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Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press Trends Towards Progress of Brains and Sense Organs* BERNHARD RENSCH Zoologisches Institut, Miinster (Westf.), West Germany to diseases; 3) higher absolute (not relative) speed in attacking or fleeing; 4) more favorable proportions of some structures growing with positive allometry, for example, relatively larger weapons (mandibles, chelae, antlers, tusks, etc.); 5) an absolutely larger number of cells of the relatively smaller sense organs and brains; 6) in mammals a relatively, and absolutely larger size of the forebrain and isocortex, that is to say, of the most complicated part of the cortex (see below); 7) more dendritic ramifications of the brain neurons; 8) correspondingly a better learning capability; 9) in warm-blooded animals a smaller loss of calories by heat radiation, as larger animals have a relatively smaller surface area than smaller ones; 10) longer duration of life; 11) in poikilothermous animals a larger number of offspring (greater number of eggs); 12) in viviparous animals relatively less burdening by young animals in the uterus because they are relatively smaller (B. Rensch 1943, 1947, 1954, and especially 1959). (In trans-specific competition, the selective value may be much higher with regard to larger differences of body size between competing species). But to some extent Cope's rule may also depend upon cumulative effects of heterosis. Normally a new race or a new species begins in a relatively small area and this means a relatively small number of individuals. Corresponding to the increasing size of the area and the increasing number of individuals, the quantity of new mutations and also the multiplicity of recombinations increases. Thus, the possibility of heterosis effects, especially the increase of body size, also increases. Such a heterosis may perhaps be the main factor, which causes the increase of body size in the lines of descent of Foraminifera where Cope's rule is valid (compare H. Hiltermann and W. Koch 1950) but where all the selective factors mentioned before seem not to be effective. If now the inherited body size is increased by one of the mentioned factors or by a combination of some of them, the proportions of most organs and structures will be shifted as they grow with positive or negative allometry with regard to body size. In such cases natural selection may also produce disadvanatgeous characters if the disadvantage is much smaller than the advantage of the increased body size. For example, it may be disadvantageous when in large animals the bones become too bulky and tusks or antlers too large. However, during the ice age it was probably of greater importance to have a relatively large body which loses relatively less heat because of CHARACTERISTICS AND TYPES OF TRENDS Many mutations primarily produce quantitative differences. They may intensify or slow down, prolong or abbreviate the development of a special structure, of an organ, or of the whole individual. Such quantitative alterations of developmental processes are finally caused by the uni-dimensionality of the course of time. When the direction of selection remains constant during a longer period of phylogeny, as is often the case, then special trends of evolution will result. Hence, we often see an increase or a decrease of the relative size of a structure or an organ or of the body size as a whole. By the addition of such purely quantitative alterations, a structure may finally reach another level of integration distinguished qualitatively by new characters or functions. In a parallel manner, the integration of pure quantitative alterations of the number of atoms leads to new molecules having new qualities. The parts of a body are connected by numerous correlations. Therefore, all quantitative alterations show many secondary effects in other structures or functions, and all trends, which are caused by quantitative mutations and by a constant selection, will cause "secondary trends", as D. M. S. Watson (1949) called them. Especially the increase or decrease of body size will shift many proportions of organs or structures and, hence, also many functions. As in many cases the process of speciation is accompanied by a change in body size, the analysis of the correlations involved will be of special interest. In many groups of non-flying animals, especially in most lines of descent among mammals, we may observe a general trend towards successive increase of body size, known as Cope's rule or Cope-D@~r6t's rule. Of course we are not here concerned with "rectilinear" alterations or with a real "orthogenesis", as G. G. Simpson emphasized (1949, 1953). It is ony a general tendency which remains constant, whereas the intensity of the alterations may increase or decrease in some periods according to special adaptations to changes of environment. Apparently this rule is caused by the higher selective value of larger varieties when they compete with smaller ones or of larger species when they compete with smaller species. Larger animals normally show: 1) greater physical strength which is important among competing embryos and young animals; 2) better resistivity * Dedicated to Th. Dobzhansky on the occasion of his sixtieth birthday. 291 Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press 292 RENSCH its relatively smaller surface area, than to have too large antlers as in Megaceros. Hence, if we evaluate the functions of an animal as a whole the giants may be called adaptive in their geological period. But when the climate became successively warmer later on, the advantage of giant size became reduced and the disadvantage of the excessive size became more apparent, and finally selection could wipe out such a species. By such a change of the selective value of different characters correlated with one another, we may perhaps explain the dying out of so many species towards the end of the Pleistocene. This short discussion of Cope's rule may be sufficient to exemplify the implications connected with correlative effects of trends. When we now try to summarize the multiplicity of trends which we mention in the lines of descent, we must confess that a classification is very difficult. We can only distinguish between general and more specific trends. The former are of much greater interest for the understanding of evolution as they govern the development of many branches of the phylogeny of animals. However, by using the term "general trend" we do not want to characterize a consistent successive trend but the sum total of many single trends, not dependent on one another, but showing the same general tendency because of their parallel selective advantages. We have already briefly discussed one of these general trends: Cope's rule, which is connected with some other rules of proportion. Another trend, which is rather common in the development of new types of morphology concerns the multiplication of equal structures. This is exemplified by the multiplication of oscula in sponges, of pharynges in Turbellaria, of proglottids in tape worms, of segments in annelids, chilopods, diplopods, of ommatidia in arthropods, of vertebrae of some fishes, amphibians and reptiles, of parts of the kidney of vertebrates, of fingers of polydactylous whales etc. Probably this trend is so common because it is rather easy to multiply by mutation a well functioning structure developed by many processes of selection during a long phase of phylogeny, and to harmonize such a multiplication with a normal functioning of the body. It seems to be more difficult to develop quite a new structure which would correspond with a larger size of the body. Rather often the trend for multiplication of homonymous structures has been followed by trend for growing heteronomy. This is exemplified by the differentiation of the first segments of Polychaeta, Crustacea anal insects or by the development of legs into mouth-organs in Crustacea and insects, or by the growing heterodonty of vertebrate teeth etc. In these cases which correspond only in regard to their general tendency, the advantage of division of labor was decisive for the development of such trends. The most general trend is probably the trend for continuous alteration, that is to say, for continuous evolution and adaptation in consequence of continuous mutation and selection. Further, one of the most important general trends is the evolutionary progress which we may state in so many lines of descent and which also caused the development of man. Evolutionary progress is characterized by one or several of the following factors (compare J. Huxley 1942, B. Rensch 1943, 1954, 1959, G. G. Simpson 1949, 1953): 1) increase of complication; advantage: increase of general efficiency and possibility of division of labor; 2) increase of more rational structures and functions; for example, by centralization of different functions; 3) special increase of plasticity of structures and functions; for example, genetical or anatomical plasticity, accommodation, etc.; 4) increase of complication and of division of labor in the central nervous systems, that means structures which are capable of especially plastic functions; 5) increase of independence of changes in the habitat; 6) increase of autonomy as a consequence of growing complication, rationalization, and plasticity. These factors of evolutionary progress are only restricted in so far as they can only contribute to definite progress. Hence, evolutionary progress does not mean improvement of a species in its special habitat (adaptiogenesis) but improvement with regard to the further development of a whole line of descent. We may distinguish this type of progress as beltiogenesis (~ehv~c0v = better). SPECIAL TRENDS TOWARDS EVOLUTIONARY PROGRESS IN SENSE ORGANS AND BRAINS In numerous special lines of descent which we know sufficiently by paleontological, anatomical, and embryological investigations, we often note a progressive improvement of the sense organs and nearly always of the central nervous system. This is especially striking in the first period of phylogeny when a new type of morphology has been developed and successive improvement takes place. Towards the end of a line of descent this improvement slackens down and at last stops, that is to say, because of the continuous operating forces of selection, a more or less favorable state will be stabilized. Thus, sense organs and central nervous systems follow the well known rule of decreasing speed of evolution, a process which J. Huxley (1957) has called stasigenesis. However, sense organs and central nervous systems have a rather different speed of evolution. Normally, the former ones show a quick improvement in the beginning of a line of descent and then remain constant for a long period. The eyes of vertebrates, for example, apparently developed quickly into a type of vesicular eyes with lens, iris, and complicated retina. The further improvement towards warm-blooded vertebrates was brought about by increasing the number of Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press PROGRESS OF BRAINS AND SENSE ORGANS sense cells and improving accommodation and adaptation. In shorter lines of descent showing much progress in other organs including brain progress as in the horse line, or in the lines of descent of carnivores and primates, the eyes apparently remained nearly stable. Or, the number of sense cells was increased because of the growing size of the eyes which we may estimate from the growing size of the orbitae. The same holds good for all other sense organs. In primitive mammals the labyrinth, nose, tongue, the organs for the sense of touch, etc., are well developed and in most cases higher mammals show only a rather insignificant improvement by increase of the number of sense cells. This increase normally corresponds to the increase of body size. For example: the small newt Triturus vulgaris has about 171,300 sense cells in each eye (diameter 2.2 mm.), while the larger Tr. cristatus (diameter of eye 2.7 ram.) has 224,300. The small salamander Salamandra atra (diameter of eye 3.5 mm.) has about 386,300 sense cells whereas the larger S. maculosa (diameter of eye 4.8 ram.) has about 533,000 (A. M511er 1950). The central nervous systems, on the other hand, show a steadier improvement in the lines of descent. While eyes, labyrinth, etc., reached a high stage of development on the level of reptiles, the brain continued to improve continuously from reptiles to lower mammals and further on to higher mammals and to man. Among fish, the forebrain is more or less a mere smelling center. In reptiles, it is relatively much larger and has several new sensory and associative functions. In mammals, the cortex was added as a new superimposed region of great complication and in several higher orders of this class new regions of association, especially in the front region, were added. Even in special lines of descent, as for example in the previously noted lines of horses or primates we may see a striking progress. In the line Eohippus-Mesohippus-Merychippus-Equus or in the lines leading from Thinocyon (Creodonta) to later genera of carnivores, T. Edinger (1948, 1956), by preparing casts of the brain case, showed that the forebrain became not only absolutely, but also relatively larger, and that the cortex increased very conspicuously by folding. The evolution from ape to man was also accompanied by marked alterations of the forebrain especially by an increase of the associative regions of the frontal and partial increase of the temporal lobes. Judging from casts of the brain case, the Australopithecinae had a frontal region which was only slightly larger than that of a chimpanzee. It is doubtful if a motor region of speech (Broca's region) in these ape men began to develop (compare Fig. 1, and W. G. H. Schepers in Broom and Schepers 1946). In Sinanthropus and in Homo neanderthalensis, the frontal lobe was much more developed. The existence of Broca's region is very probable 293 FIGURE 1. Lateral view of forebrains. Equally diminished in size. Above: chimpanzee, middle : reconstruction (after cast of brain-case) of Plesianthropus (Australopithecus), below: of a negro. Motor praecentral region black, frontal brain (mainly associative centers) dotted. Broca'region (and possible prestage in Plesianthropus) densely dotted. (After G. W. H. Schepers from B. Rensch 1959a). but the basal cortex of the frontal lobe was only slightly folded compared with the structure in Homo sapiens (H. Spatz 1950). Judging by findings of pathologists, this basal region is especially important for the consistent series of hypotheses and for creative performances of man (K. Kleist 1934). We may also state different trends with regard to the histological differentiation and division of labor among different regions in the forebrain of vertebrates. The undifferentiated dorsal brain tissue of Agnatha became a tripartite pallium in fish and amphibians, the lateral regions of which were changed into the neocortex of mammals. During the phylogeny of mammals, the relative extension of the isocortex, that is to say, of the most complicated 5- and 7-layered cortex was enlarged and divided into more and more special functional regions. Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press 294 RENSCH Parallel to this development, a growing number of special cells originated. Besides, the pyramidal, granular, and star cells already known in Anamnia and the giant star cells already existing in reptiles, later, compass and bifurcated cells arose in mammals (still lacking in Insectivora; A. Syring 1957). Apparently most of these types of cells have the function of integrating excitations in a special manner. Will it now be possible to analyze these different trends of progression in vertebrates by quantitative statements? This seems to be the case although normally we cannot study the lines of descent themselves. As already mentioned, there are only a few lines like that of the horse where we may estimate the alteration of the superficial brain structure, especially of portions of different parts and of the main foldings from casts of the brain case. However, we may use a series of brains of recent species as models of lines of descent. We may compare the absolute and relative size, and thus the progress of the whole brain, of the forebrain or of single regions in the "ascending line" of vertebrates or of mammals only; for instance of Monotremata, Marsupialia, Insectivora, Carnivora, and Primates. Of course, this method is not quite correct. One may object that the socalled primitive types of recent species may be primitive only in some characters of the brain and that other characters may be rather progressive. There is no doubt that this is the case in many species. The Monotremata, for example, have a relatively large forebrain and the kangaroos show a fair learning capacity (see below). However, if we restrict our conclusions to some quantitative relations this method of comparing series of recent animals can be correct. If we are capable of establishing general rules valid for many related species differing only in absolute body size or absolute brain size, then we must assume that these rules were already valid in the common ancestors of the compared species and hence also in the lines of descent of large species normally beginning with smaller species (Cope's rule). By such comparisons it has been possible to note a certain relation between brain size (E) and body size (C) which may be expressed by the formula E = b 9C a. In this function formula, the allometrical exponent a indicates the relative growth of the brain in series of increasing body size. For mammals, different but similar values have been calculated. By comparing many species of different orders E. Dubois (1898, 1930), L. Lapique (1898, 1907), and R. Brummelkamp (1946) found a = 0.56, D. P. Quiring (1938) found 0.58 for African ungulates, G. yon Bonin (1937) who investigated many species of different orders of mammals found a = 0.66, H. J. Jerison (1955) a = 0.67, and O. Snell (1891) a = 0.68. Apparently these differences depend on differences of preparation and weighing, and also on differences between carnivorous and herbivorous, or between quicker and slower, or burrowing and free-living, species. Hence, D. Sholl found statistically significant differences between two families of rodents: in Sciuridae he calculated a = 0.60; in Muridae a = 0.51. But in spite of such differences we can say that in mammals, the phylogenetical increase in body size is accompanied by an increase of the brain which is more or less proportionate to the surface area of the animals (a = 0.60). This rule seems to be related to the fact that the innervation of the epidermis and many interior organs depend upon the surface areas. Considering these allometrical exponents it becomes possible also to find a formula for the relation between brain and body size or weight, indicating approximately the level of phylogenetical progress of a brain. Perhaps the best formula was developed by R. Mfiller (based on E = b. C~ brain index brain weight3 - body weightv In another manner K. Wirz (1950) calculated the level of "cerebralization" by a "neopallium index", which does not satisfactorily reflect the level of progress in all cases. This author divides the brain weights by an artificial basic figure which would indicate the weight of the brain stem if the animal in question belonged to the Insectivora. However one may object that the brain of ancestral species of ungulates, carnivores, and other groups did not correspond to the brains of recent Insectivora in the Early Tertiary, but showed a more primitive, reptile-like structure. Of course, the absolute and relative size of the brain or of the forebrain is not yet a sufficient criterion for the phylogenetical level. The histological structure, too, is of great importance. We now know that in mammals the density of cortex cells decreases with increasing absolute brain size. S. T. Bok and M. J. Van Erp Taalman Kip (1939) could show this in rodents of different body size, and D. B. Tower (1954) described it in mammals of other orders from mouse to elephant. The latter author stated that the decrease can be expressed by the equation N = KN 9W R in which N indicates the number of nerve cells per unit of volume; KN means a constant; W, the brain weight; and R, the coefficient of regression of the species in question. Corresponding to this rule the activity of the acetylcholine system which is parallel to the volume of nerve cells shows the same relation to the brain size. On the other hand, larger species of vertebrates nearly always have larger neurons showing a much richer ramification than related smaller species (S. T. Bok 1936, B. Rensch 1949, A. Spina Franca Netto 1951). Hence, in absolutely larger brains more possibilities of connections of fibers and associations exist which is an advantage and which may contribute to brain progress. Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press PROGRESS OF BRAINS AND SENSE ORGANS 295 TABLE 1. RELATIVE SIZE OF THE SURFACE (IN PER CENT OF THE SURFACE OF THE WHOLE FOREBRAIN) OF 4 DIFFERENT REGIONS OF THE CORTEX OF WHITE RAT AND WHITE MOUSE AND OF GIANT SQUIRREL ( R a l u f a ) 51 ""q" ,w ~o 2'0 io ,~o go 6o 7b sb 9'o,~,~ AND DWARF SQUIRREL (Funambulus) In parentheses: number of analyzed hemispheres. (After K. W. Harde and CH. Schulz). Muridae 39 38 35J ~ . . Io z0 (12) white (6) white (3) Ratufa (4) Fur~indica ambulus rats mice palmarum ........... ""'l" i0 ~,o i0 6o 7b Sciaridae 8o 4 0 d ~ FIGUHE 2. Changing relative size of the sehizocortex (above) and the holocortex 7-stratificatus (below) in per cent of the whole surface area of the hemisphere (ordinate) during the postnatal ontogeny of the white mouse. Wedge = day of the opening of eyes, hatched part = beginning maturity (after K. W. Harde). Still more important is the fact that the cytoarchitectonieal structure of the forebrain is altered corresponding to increasing brain size. This alteration is advantageous. Of course, in this case, we can only estimate phylogenetical alterations by comparing related recent species of different body size and brain size. However, as already mentioned, we are entitled to use this method. The author has repeatedly summarized (last in 1958) these investigations carried out in the Zoological Institute of the University of MSnster for many years. Hence, I may restrict myself to a short outline of the main results with regard to the cortex of mammals. By measuring a large number of sections of the forebrain of mice, K. W. Harde (1949) could show that most regions and smaller areas grow in a different manner with positive or negative allometry in relation to the whole cortex. In some stages a change in the direction of allometry takes place, for instance, after birth, at the period of the opening of the eyes, or at the beginning of maturity (Fig. 2). Hence, when allometrieal exponents remain more or less constant during phylogeny, related species of different body size show different proportions of cortex regions. In some eases the single allometrieal exponents have been altered markedly during phylogeny apparently in consequence of special selection caused by special modes of life. But in most eases the general allometrical tendency remained the same and only the value of the allometrical exponents increased or decreased but did not change from positive to negative allometry. And especially one very important tendency remained unchanged: the relative enlargement of the isoeortex, that is to say, of the most complicated and most progressive 5- and 7-layered regions. This enlargement took place partly at the cost of phylogenetically older regions, like the semicortex or schizo- Average weight of animal.. 214 g 26 g 1237 g 111 g Isocortex Bicortex Schizocortex Semicortex 53.6 23.1 9.6 13.9 49.4 22.9 9.0 18.7 70.5 14.4 6.3 9.2 62.4 18.2 6.6 12.8 J TABLE 2. RELATIVE SIZE OF THE SURFACE (IN PER CENT OF THE SURFACE OF THE WHOLE FOREBRAIN) OF 4 DIFFERENT REGIONS OF THE CORTEX OF 4 BATS OF DECREASING BODY SIZE (After F. Ltitgemeier) Pteropu medius aulti Average weight of animal.. 723 g 85g Isocortex Bicortex Schizocortex Semieortex 63.9 18.4 5.0 12.7 53.8 23.3 6.0 16.9 serotinus daubentoni /_ 20g 93.8 34.2 9.7 16.3 8,5g 37.4 33.6 12.9 16.1 cortex. Tables 1 and 2 show these differences of relative size in rodents (K. W. Harde 1949) and in bats (F. Luetgemeier). As this rule seems to be valid for species of different body size in 4 families of 2 different orders of mammals and as it also proves right in Primates (if we compare monkeys, apes, and man), and in mammals as a whole, (if we compare more primitive with more progressive orders), we may assume that we have to do with a general rule. It may be explained by the greater selection value of the more complicated cortex. This histological complication allows more complicated connections of nerve fibers, more complicated associations, a richer memory, and hence a more plastic behavior which is better adapted to different external situations. Summing up, we may agree that a progressive trend in the evolution of the brains of vertebrates is characterized by the following advantages: 1) increase of absolute brain size, in longer lines of descent and increase of relative brain size; 2) increase of the histological complication and of the division of labor of the forebrain, which finally produced the multilayered cortex of mammals; 4) increase of the 5- and 7-layered Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press 296 RENSCH isocortex; and 5) of the special regions of association in mammals. In this context we must bear in mind that by the increase of the number of neurons the possibilities of associative connections will increase in a geometrical progression. Between 4 neurons, 6 different possibilities of mutual connections exist, between 8 neurons, 26, and between 16 neurons, 120. Of course, it will be impossible for each neuron to come in contact with each other one but this fact does not alter the general statement that the possibility of connections and the degree of functional capabilities will increase very markedly by each increase in number of neurons. However, only in the course of long lines of descent, the progressive increase of the brain of vertebrates was accompanied by a noticeable increase of the number of neurons. This is seen in the lines of descent leading from the Creodonta of the Eocene to recent Carnivora, from lemurs to monkeys, apes and man, and in the lines leading from the Insectivora of the Cretaceous to all higher orders of placental mammals. In shorter lines of descent, the number of neurons remains more or less the same because, as already mentioned, the density of cells decreases along with the increasing brain size. However, parallel with the increase of cell size, the number of dendritic ramifications will be increased (S. T. Bok 1936, B. Rensch 1949, G. A. Shariff 1953, A. Spina Franca Netto 1951), and therefore the capability of more complicated connections and associations will increase very markedly. Now, it is an important task to find out in which manner better nervous functions run parallel with the increase of brain size or size of the forebrain and also with the increase of histological differentiation, especially with the increase of the 5- and 7-layered isocortex. There can be no doubt that the brain functions of higher mammals such as carnivores, monkeys, and apes having an absolutely large folded forebrain, show a much greater multiplicity than the brain functions of lower mammals with unfolded forebrain as, for example, rodents and marsupials. Correspondingly the brain functions of birds show a much greater multiplicity than the brain functions of reptiles or amphibians. However, fish have a small brain and a forebrain which functions only as a center of smell, but show much better capabilities than amphibians and perhaps some reptiles, too, although the latter have a relatively large forebrain. Trout, which were used in the experiments of A. Saxena who worked in our Institute, were capable of learning 6 visual tasks, discrimination between two colors or two patterns in black and white; they could solve these problems in multiple tests (Table 3). Such fish were able to retain a pair of patterns of cross against circle more than 80 days, and two colors (red against green) more than 150 days. There are no reports of similar capabilities of amphibians and reotiles. Lizards T A B L E 3. RESULTS OF A M U L T I P L E T E S T IN TROUT Percentage of correct choices of 3 specimens having learned 6 visual tasks. Average of 60 trials for each task for each fish. (After A. Saxena). Tasks 1 Trout 1 Trout 2 Trout3 2 3 4 5 6 83 81 78 81 81 88 78 86 81 83 81 81 [ 86 I 85 I 83 / 86 I ~ I 90 trained by H. Wagner (1933) and by H. Ehrenhardt (1937) were not able to learn differences of colors or patterns in black and white. (C. Hinsberg, working in our Institute, seems to get better success in training young iguanas and turtles.) R. J. Wojtusiak (1933, 1934) succeeded in training turtles to discriminate between pairs of colors and geometrical patterns. Hence, we may doubt that a purely quantitative increase of the brain or the number of neurons may raise the learning capability. However, a correlation between brain size and learning capability becomes very probable from other experiments which we performed in the Zoological Institute of Mfinster during recent years. We trained related species of markedly differing body size and hence very different brain size, to learn the same tasks in order to find out whether or not the larger species or races showed better capabilities. We could state that generally the learning capacity of larger species was better than the capacity of related smaller species. White rats were able to learn successively 8 visual tasks which they mastered in multiple tests whereas white mice mastered only 6 tasks (W. Reetz 1957). An Indian elephant mastered 20 visual tasks at the same time (B. Renseh and R. Altevogt 1955), a horse, 20 tasks, a donkey, only 13, and a zebra, only 10 (H. D. Giebel 1958). A large race of domestic fowl, the brahmas, mastered 7 pairs of patterns in multiple tests, medium-sized races, only 5, a dwarf race, 4 to 5 (R. Altevogt 1951). As already mentioned, trout (Trutta shasta) mastered 6 similar visual tasks in multiple tests (A. Saxena), small Cyprinodontidae (Lebistes and Xiphophorus), only 2 or perhaps 4, and not in a statistically significant percentage (B. Rensch 1954b). We found corresponding differences of larger and smaller species or races when we tested the time of retention. The best of the rats trained by W. Reetz solved one of 8 learned tasks after 459 days, whereas the best of her mice retained one of 6 learned tasks only 195 days. The giant race of fowl of R. Altevogt solved all the 6 trained tasks after an interval of 20 days in a statistically sufficient percentage, while the dwarf race retained only 3 to 5 tasks. The trout of A. Saxena mastered Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press PROGRESS OF BRAINS AND SENSE ORGANS a single visual task (two black and white patterns) after 80 days, another task (colors) after 150 days without training. The larger Cyprinodontidae which I had trained myself retained a visual task for about 54 days on the average. It is not certain that larger animals have a better capability of recognizing a learned pattern when the size or the color is altered or when the pattern is transformed in such a manner that only partial components remained the same or that only a relation between two patterns (for example larger-smaller) remained unchanged. In this respect rats are apparently a little better than mice, giant races of fowl better than dwarf races (experiments of W. Stichmann under way in our Institute), trout better than Cyprinodontidae. However, the results are not yet sufficient for a generalization to be made. Summing up, we may establish a working hypothesis based on the following facts: larger vertebrates have a greater learning capacity and a longer capability of retaining visual tasks than related smaller species or races with absolutely smaller brain, having a similar mode of life. There can be no doubt that these capabilities of larger animals (larger brains) have a positive selection value, which has determined the tendency of evolutionary development. It is rather probable that it was, at least partially, a victory of the better brains when, in former geological epochs, the Agnatha were replaced by competing higher types of fishes, marsupials by placental mammals, Creodonta by carnivores, and lemurs by monkeys. In many lines of descent natural selection caused an increasing enlargement and improvement of the brains and therefore it favored the increase of the correlated body size, especially by interspeeific competition for food and habitat. Thus it contributed to formulation of Cope's rule. SPECIAL PROGRESSIVE TRENDS IN SENSE ORGANS AND BRAINS OF INSECTS In order to generalize upon the results found in vertebrates, it is necessary to carry out corresponding investigations in invertebrates, especially in groups showing a complicated brain. It became obvious therefore that differences in the brain of related larger and smaller species of insects should be analyzed and that the allometrical tendencies during ontogeny should be studied. In this case we must bear in mind that we cannot compare actual lines of descent, and that the size of the brain capsule of fossil forms does not indicate the brain size because the size of the head depends to a large degree upon the relative size and special organization of the mouth parts and of the eyes and antennae. Hence, the brain capsule may be filled out with brain to a very different degree as modern species will show. However, we may consider the recent species of the order of Blattaria, an order existing since the Carboniferous, as representatives of a primitive 297 type, and the species of Hymenoptera, Coleoptera, Diptera, etc.--that is to say of orders developed in the Mesozoic--as more advanced types. Furthermore we have to consider that a phylogenetical increase of body size did not take place (or did so only to a small extent) in insects. As these animals have an external skeleton, large species had to develop an excessively thick skeleton which would cause a negative selective value. On the other hand, the capability of flying also restricts a possible increase in body size, for the body weight grows by the third power whereas the surface area and hence the function of the wings grows by the second power. However, in spite of these restrictions we can state certain rules of correlation between body size or brain size and special brain structure, if we compare related species of a similar mode of life. And these rules show several parallelisms with the rules of vertebrates. We may also obtain corresponding results if we compare the sense organs of smaller and larger species. This becomes especially conspicuous by a comparison of the number of ommatidia. A few examples may be sufficient. The tiny beetle, Bryaxis haematica (body length 2 mm.), has only 32 ommatidia whereas the giant beetle Archon centaurus has 29,450 (K. Leinemann 1904). Drosophila melanogaster ( 9 ) has about 668 ommatidia; the large fly Calliphora erythrocephala ( 9 ) 4,651 on the average; the tiny gallmidge Contarinia (o~) 140; the much larger Culex pipiens (c~) 488; the still larger Tipula oleracea ( 9 ) 1,848 on the average (W. Partmann 1948). The numbers of smelling cones and of sensitive bristles on the feelers of comparable larger and smaller species show similar large differences (for example Melotontha compared with Phytlopertha or Geotrupes compared with the small species of Aphodius). If we now compare the relative size of the more important structures of the protocerebrum we may see certain trends which are correlated with growing or decreasing body size. Detailed investigations of the brains of many species of Blattaria, Hymenoptera, Diptera, and Coleoptera (H. Goossen 1949, R. Neder, W. Hinke, E. Bertram), carried out in our Institute, show that larger species generally have relatively larger and more differentiated corpora pedunculata in which the surface of the neuropil is more folded. In a similar manner as in vertebrates it is the most anterior part of the brain which is mainly enlarged. The corpora pedunculata may be looked at as the histological basis of many instincts and of learning processes because they are much more developed in insects with complicated instincts, like the social Hymenoptera. But contrary to the differenees among vertebrates, the enlargement is not caused by an enlargement of the neurons but by an increase of the number of neurons, in the ease of the so-called globuli cells (Table 4). However, the central body, the lobes and the pons of the Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press 298 RENSCH neurons in insects has been caused b y the rich development of sense organs, especially of eyes and of organs of smell. Thus, m u c h more complex patterns of excitation arose in the sense organs and these patterns were favorable only if they could be transferred in undiminished complexity to the brain. This was possible only by increasing the n u m b e r of neurons there. I n this manner, the brain could respond to single details of a pattern of excitation. As the size of an insect is limited by the tolerable size of the head and as there arose an enormous number of sense organs, the neurons of the brain had to become very small. We can find the same tendency of decrease of sensorial cells in the brain in cephalopods and in vertebrates, t h a t is to say, in groups of animals showing a similar increase and improvement of sense organs. Of course, the growth ratios determining the differences in the brain of imagos of large and small species are dependent to a large extent upon the differences of the larval development. I n Blattidae, t h a t is to say in a primitive hemimetabolic group, R. Neder, working in our Institute, stated t h a t the corpora pedunculata grow with positive allometry up to the third larval stage, later on up to the last stage (the 6th in Phyllodromia, the 10th in Periplaneta) or to the imago with negative allometry (Fig. 3). However, in holometabolic insects, the larvae of which differ very much from the imagos, the corpora pedunculata grow with positive allometry during the whole larval development (Apis: E. Bertram), or protocerebrum and the lobi optici are relatively smaller in large species. I t is of great interest t h a t the Polychaeta show quite different tendencies. R. B. Clark (1957), who investigated the brains of 17 species of very different body size in the genus Nephtys (5 to 300 m m . long), stated t h a t the number of neurons was always the same, whereas the size of the neurons was different in proportion to brain size. Hence, the totally different tendency of insects was a new phylogenetical acquisition. Probably the new trend of increasing the number of brain TABLE 4. NUMBER OF NUCLEI OF GLOBULI CELLS IN A SECTION (THICKNESS 10 p) THROUGHTHE CENTER Of CORPORA PEDUNCULATA Differences of related species of different body size. (After H. Goossen). Body length in mm. Carabidae 10phonug pubescens c~ 1 Agonum muelleri 9 Dytiscidae 1 Dytiscus marginalis Q 1 Ilybius fenestratus c~ ,~carabaeidae 4 Geotrupes stercorarius 1 Aphodius fimetarius 1 Melolontha vulgaris o~ 1 Phyllopertha hosticola Number of nuclei 11.6 6.2 614 208 27.0 12.0 615 188 17.3 6.5 20.6 9.9 247 101 1776 624 % 3~ o 32 30 28 26 2~ 22 20 18 16 12 I0 I I I I I I I 0.I 0,2 0.3 0.~ 05 0,6 0.7 I 0.8 mm3 FIGURE 3. Volume of the corpora pedunculata in percent of the volume of the whole brain (ordinate) in growing cockroaches. Crosses ( 9 ) and stars (~) = Periplaneta americana, white ( 9 ) and black (o~) circles = Blatta orientalis, white (9) and black ((~) triangles = Phyllodromia germanica. (After R. Neder). Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press PROGRESS OF BRAINS AND SENSE ORGANS they grow with negative allometry nearly during the whole larval period. Only shortly before the development of the pupa in Culex or during the early pupal period in Drosophila they grow with strong positive allometry (Fig. 4; W. Hinke). As there is almost no further growth after the pupal period, it was necessary to chart the number of hours on the abscissa in Fig. 4. Hence, the graph shows that after quick growth of the corpora peduneulata in the pupal period, the definite portions of the brain are developed and remain constant. In Myrmeleo, an insect which has a rather active larva, the corpora peduneulata are developed rather early. The central bodies, on the other hand, grow with strong positive allometry since the third larval stage in bees, or after the development of the pupa in Culex and Drosophila. In the larvae of Drosophila, we may also estimate the correlations between the corpora peduneulata and the lobi optici by comparing the larval development of mutants with reduced eyes (Bar % u\ ',n ', '..,. ".. "-. A~ m ~o w 9 " 9 ,A . . . . . . . . . . . A . ~ _-.-, . _ . A ..u .... ,i s~ , [.,' ~..' 9 FIGURE4. Above: relative growth of the corpora peduneulata; below of the lobi optiei in per cent of the brain in 3 strains of Drosophila melanogaster. Abscissa: hours of development and stages of larvae, pupa and hatched imagos. IH and II H = 1. and 2. moult of the larva, AL = old larva, V = time of pupation, KA = turning out of the head, Sehl. = hatching. ----wild - - - B a r ..... eyeless 299 and eyeless) with normal flies. Such investigations are of special interest because they show the developmental effect of a single mutation. Thus, we may analyze one of the chains of reaction which are controlled by the allele in question. Besides, the correlations which we may observe in such a manner show some of the pleiotropic effects. One of my students, W. Hinke (not yet published) showed that in such mutants the lobi optici are not developed to a normal extent during the pupal period. This alteration also affects the development of the corpora peduneulata. The mutant Bar shows a marked decrease of the positively allometrieal phase of the corpora peduneulata. In the early pupal period and in the mutant eyeless, the corpora peduneulata grow only isometrically in this period (Fig. 4). With regard to these differences in the main developmental phase of the corpora peduneulata as seen to exist between hemimetabolic and holometabolic insects, it becomes conceivable that giant species of both groups show different tendencies of the brain. The Australian giant cockroach Maeropanesthia rhinocerus does not show, contrary to the above mentioned rule of body size, an increase in the relative size of the corpora peduneulata compared with Periplaneta americana, the body of which has only ~ 2 the weight of Macropanesthia. Apparently the phylogenetic development of this giant species took place through prolongation of the last larval phase or by addition of new larval stages at the end of the phase which shows a slightly negative allometry of the corpora pedunculata. It is a pity that the ontogenetieal development has not yet been studied. C. Ratzendorfer (1952) tried to determine the levels of progress of insect brains in a similar manner as in vertebrates, that is to say, she calculated the higher differentiated parts of the brain as per cent of the less specialized remainder of brain parts. This corresponds more or less with the usual method of characterizing the phylogenetieal level by the relative size of the corpora pedunculata. Thus, Ratzendorfer could show that progressive brain types may be found anaong holometabolie as well as among hemimetabolie insects. Unfortunately, differences between larger and smaller related species of insects with regard to the capacity of learning or retaining have not yet been carried out sufficiently. Extensive experiments with Carabidae carried out by the author, showed a better capability of retention for some larger species, but the results were rather mixed in kind. However, it is probable that the abundance and the complication of instincts are much greater in larger insects (that means in insects with absolutely larger brains) than in smaller related species. All large species of European bees and wasps, for example, show very complicated social instincts, whereas the small species of the same families are solitary. The Searabaeidae show Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press 300 RENSCH corresponding differences. Scarabaeus, Copris, Geotrupes and other large species have much more complicated instincts for care of eggs than related small species of the genus Aphodius. Among Silphidae, Necrophorus has much more complicated instincts for the care for eggs and larvae than the small Catops. GENERAL CONCLUSIONS From the investigations reported above we may conclude that a purely quantitative increase of the structure of brains and sense organs can produce evolutionary progress especially by increase of better central nervous and parallel psychic performances. Plus mutations causing such a growing intensity or a prolongation of the growth ratios of sense or brains are always possible and are rather common. They can be advantageous because an increase of sense cells or brain neurons enables an animal to react to finer details of a pattern of excitation. When in primitive Arthropoda, compound eyes began to develop during phylogeny they were able to discriminate only between dark and light and between directions of light, as long as the number of ommatidia remained small (as in Cladocera and Copepoda). Only a strong increase in the number of ommatidia, as this is exemplified by higher Crustacea, allowed the discrimination of details of a pattern of excitation and thus discrimination of details of a pattern of excitation and thus a visible reaction to prey, to enemies, etc. The same holds good for the neurons of the brain, the increase of which enables an animal to answer to complicated patterns of excitation in more detail, to develop more plastic reactions and a more complicated memory. As all such alterations had a positive selective value they automatically caused an evolutionary progress in many lines of descent. In most cases a correlation between increase of brain size and increase of body size existed (Cope's rule) and the latter was often advantageous, too. If now a new additional brain region arose by mutative increase of the positive allometry of a neighboring region, a new combination of excitation flowing in from all neighboring regions took place. In such a manner new functions could arise and a special selection of these new functions could begin. A typical example for such a development of a new level of integration is the evolution of the motor center of speech in human brains, of Broca's region. Here we have an additional region of the lateral frontal brain which neighbors the regions for tone memory, for the movements of mouth and tongue, and for the motor impulse of complicated associations centered in the basal part of the frontal lobes. Hence the newly developed region was more or less predetermined for becoming a motor center for speaking. However, as soon as the new function, speech, began to develop this new character of man became so important, because of the possibility of more abstract thinking, the mutual exchange of experience, and the development of tradition, that now a special selection quickly favored the further evolution of Broca's region. Similar additions of brain regions primarily without a special function seem to have occurred several times during the evolution of the brain of recent mammals. They had a positive selection value because they allowed an increasing division of labor. As now the main functions of the brain, especially the well differentiated centers of sense, existed on more primitive phylogenetical levels, normally the larger additional regions could only function as co-ordinating and integrating centers on a higher level. The basic evolutionary process, however, was only the appearance of plus mutations favoring certain growth ratios and secondarily allowing new processes of special selection. A few examples may illustrate this. The forebrain of vertebrates became relatively larger in the course of evolution. In amphibians and still more in reptiles it became the largest part of the brain. Its functions, however, still remained rather restricted. The learning capacity and the memory of amphibians and even of reptiles is very limited although the latter show more histological differentiation which includes a visual center. In mammals, the forebrain becomes the dominating brain part as it co-ordinates many excitations of other parts. In the forebrain, the cortex becomes more and more complicated and gains the function of co-ordination and finer evaluation of subcortical excitations. In lower mammals, the whole cortex is divided in several fields of projection for different sense regions. When later, the cortex became still more enlarged, the new regions, especially in the front part and in the temporal lobes, could assume again a new directing function: they became regions of association in which the excitations of the same regions were combined and evaluated on a higher level. Finally, during the evolution of the genus Homo, Broca's region of speech was developed which facilitated not only speaking but also thinking in words in a more abstract manner, as mentioned above. On the other hand, the basal part of the frontal brain became more and more folded (in Pithecanthropus it was unfolded: H. Spatz 1950) and here a region arose acting as a kind of impulse-giving motor for complicated associative thinking. The development of these typically human brains probably based on purely quantitative mutations, was perhaps the decisive event in the origin of Homo sapiens. It may be that the origin of these regions stopped the rapid increase of the brain, because the accumulation of tradition replaced the selection pressure for pure increase in size. All the mentioned examples show that a new region, developed by enlargement of neighboring regions, may secondarily get new functions. In such cases we may speak of a postintrogression of functions. Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press PROGRESS OF BRAINS AND SENSE ORGANS Summing up, we m a y state once again that the improvement of sense organs and of brains which is characteristic of the evolutionary progress, may be explained by considering only quantitative basic alterations, that is to say undirected plus mutations which determine the growth ratios and which m a y be favored by selection. Such quantitative alterations did not only increase the corresponding functions but also caused the development of quite new levels of integration. Because of the systemic laws of correlation, this integration m a y also lead to new qualitative characters. As quantitative plus mutations m a y arise at at any time and as advantageous varieties are favored by selection (favorable structures of brain, especially by interspecific selection, for example: placental animals competing with marsupials), a progress of brain structure in different lines of descent must result. Here we touch on the important question of how far the origin of man was necessitated. In our context it will be impossible to treat this special problem and I will refer only to the corresponding short discussion in m y little book on H o m o s a p i e n s (1959a). On the psychic side, new levels of integration have been developed successively. However, they do not run parallel morphological levels as closely as one could expect. As already mentioned above, the morphological dominance of the forebrain in amphibians and even in some reptiles did not produce better performances than shown by fishes, and the development of a cortex in lower mammals likewise did not produce better performances than those of birds. This lack of parallelism became evident in the learning capacity of different classes of vertebrates. M y coworkers always used the same or very similar patterns for training different animals. An opossum was capable of learning one pair in a pattern, a giant kangaroo, however, mastered six pairs in a pattern in a multiple test (Neumann), white mice mastered six pairs, rats 8 pairs, a donkey 13, a horse and an elephant 20. Domestic fowl without true cortical regions also learned 6 to 7 similar tasks, and even trout, working only with the midbrain, mastered 6 tasks in a multiple test. Hence, I have the impression that the learning capability and the psychic performances on the whole depend more upon the number of brain neurons and on the size of the associative brain parts than on the histological differentiation, although the latter is important, too. The special psychic levels of integration leading to the origin of man m a y perhaps be characterized by the following succession: increase of free choice based on experience (parallel: reduction of pure instinctive performance); performances by insight (foresight, thinking by including possible future performances) knowledge of causal connections, abstract thinking by the help of words, knowledge, and understanding of the laws of the world. Wrong ways of evolution, that is to say wrong thinking and acting has always been 301 corrected by natural selection. This means the whole evolution of thinking and other psychic processes also show a successive adaptation to the laws of the world. REFERENCES ALTEVOGT,R. 1951. 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Lett. 349-373. DISCUSSION EMERSON: These investigations of the evolution of brain function should be continued and extended, but I think it m a y be too early to generalize from the insects. In the case of the termites, progressive evolution of social behavior is predominantly associated with a decrease in size, the most primitive forms being large, and the majority of the derived forms being small. We have very little data on the functions of the central nervous system of insects, and generalizations from brain size of vertebrates with highly developed capacity for associational learning across to organisms with a high degree of instinct is questionable. RENSCH: I t is a pity that we do not know enough about the brain structure of termites. Hence m y considerations could only be based on European bees, wasps, and beetles. I n European species of bees and wasps only the larger species show complicated social instincts. I know well that there exists also rather small social Hymenoptera in the tropics. I t would be of great interest to investigate the brain structure of these animals. I t could be possible that the number of brain cells is very large although the brain as a whole is absolutely small. I n ants, too, the smallest genus, Monorium, shows the simplest instincts. On the other hand there are large species like in Camponotus having comparatively simple instincts. I would not say that the complication of instincts in insects always runs parallel with absolute brain size. But I believe that species with small brains and relatively few brain cells normally have less complicated instincts than related and biologically comparable larger species, which m a y have more complicated instincts or m a y also have simpler instincts if their mode of life does not require a complicated behavior. Of course this can only be a working hypothesis. CURTIS: I wish to congratulate Dr. Rensch and his collaborators on a fine piece of work. I hope that their efforts will continue in their present direction. M a y I ask for clarification of the following point. The exclusive use of serial visual discrimination tasks in most of the studies suggests a possible confronting of learning ability and visual acuity. Larger animals presumably have more perceptor elements in the retinal fovea, and m a y have better resolution of visual patterns. If so, the tasks are not of equal difficulty for large and small animals. Auditory, tactile, and propriocep- Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press PROGRESS OF BRAINS AND SENSE ORGANS rive discrimination suggest themselves. Have you used other discrimination learning tests? You mentioned the oddity problem. Have you systematically studied size difference by means of Horten's delayed response found by Pribram and by Harlow to be a very useful measure of comparative brain function. RENSCH: YOU are right in presuming, that larger animals have more sense cells than closely related smaller species. We could show this by countings in the retina of newts and salamanders. However, the difference is not very great and probably does not affect the visual acuity. For example in our experiments, the learning boxes and the distance of the animal from the learned pattern were more or less proportionate to body size and the size of the patterns proportional to the diameter of the eye. We have not yet made experiments with delayed responses but one of my collaborators who performs transposition experiments with large and small races of domestic fowl, will do it later on. SKERL:Working on a paper on human evolution I 303 found in Matiegka's book (1937) a quotation which supports the graph showing the increasing frontal part of the brain ill a modern Negro vs. the Australopithecus. It is, according to Well, that the surface area of the brain in front of the vertical line touching the frontal edge of the temporal lobe, was in Neanderthalers 21-25 per cent only, whereas in Modern Man it is 28-31 per cent of the whole brain surface area. VAN VALEN: DO you know whether, or how much, the relationship of brain size to learning ability in related animals is due to the number of neurons, as might be expected, or to their size? RENSCH: Among mammals, especially in rodents, the number of brain neurons seems to be very similar in related smaller and larger species. Especially the Dutch histologist Brummelkamp made corresponding countings. However, larger species have larger neurons with more dendritic ramifications and this may allow more complicated patterns of excitations and more complicated associations. Among insects the larger species have many more neurons than related smaller species. Downloaded from symposium.cshlp.org on February 19, 2016 - Published by Cold Spring Harbor Laboratory Press Trends Towards Progress of Brains and Sense Organs Bernhard Rensch Cold Spring Harb Symp Quant Biol 1959 24: 291-303 Access the most recent version at doi:10.1101/SQB.1959.024.01.027 References This article cites 35 articles, 1 of which can be accessed free at: http://symposium.cshlp.org/content/24/291.refs.html Email alerting service Receive free email alerts when new articles cite this article sign up in the box at the top right corner of the article or click here To subscribe to Cold Spring Harbor Symposia on Quantitative Biology go to: http://symposium.cshlp.org/subscriptions Copyright © 1959 Cold Spring Harbor Laboratory Press