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10 Divergence from Type
Type may be defined as the mean of the hereditary beneficial attributes established by natural selection in a species. Variation from type is a thing of constant and regular occurrence. Nature, in the popular saying, never produces two things exactly alike. It will be found, however, that variation in any species, if it can be measured at all, generally exhibits a regular curve. The Dutch botanist Hugo de Vries found, for instance, in measuring the length of a number of seed-pods, that the majority of pods fell under the number nearest the mean. The absolute mean contained the greatest number and as the extremes were approached they became regularly less - on the one side the long ones and on the other the short. And this law holds good throughout nature. It is, in fact, no more than a mathematical demonstration of the concrete truth upon which rests our abstract conception of type.
Theoretically, one would assume that the tendency of natural selection would be continually to eliminate the more extreme variants. Individuals possessing in the most perfect form the selected characteristics would be the ones most likely to survive, and these would be typical individuals. This theoretical interpretation of the method of selection has also been empirically verified. The American naturalist Hermon Carey Bumpus found, on measuring birds of one species that had been injured during a storm, that the individuals that perished were the extreme variants. Those that recovered were the individuals nearest the mean. Weldon and Tower found that the same thing occurred in beetles and terrestrial molluscs. Nor could it well be otherwise. If the deathrate was not highest among the variants, the theory of natural selection could not stand.
It is, of course, conceivable that variations may sometimes, under favourable conditions, prove more beneficial than the typical attributes, and it is possible that such discontinuous variations or mutations may, as Professor Hugo de Vries assumes, occur at long intervals, and that these may be the origin of distinct varieties and species. Whether this be so or not, it seems certain that favourable variations are the exception in nature. The general rule is that divergence from type is the mark of the unfit. In this country instances of extreme variation among the larger mammalia occur very rarely, but they are not unknown. In such cases, it would seem, sexual selection tends to become the dominant factor in the elimination. Albinism is an instance of such extreme variation. Several naturalists have noted that albino birds, kept under observation, did not succeed in finding mates, and we have trustworthy records of the same thing being observed among the larger African mammalia. Thus an albino springbok (Antidorcas marsupialis marsupialis) was solitary. Whenever she
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came in contact with a troop of her own kind, she was mercilessly attacked by male and female alike. A white klipspringer (Oreotragus saltator) was also so continually persecuted by other klipspringers that it took shelter near a homestead and became quite tame. It was never seen with a mate. A black wildebeest (Connochaetes gnou) with a white blaze18 was also thrust out of the herd, which it always followed at a safe distance. A fur-horned kudu (Strepciceros strepciceros strepciceros) was solitary, but this may have been due tot the continual pursuit of hunters who were anxious to secure so unusual a trophy. It would thus seem as if natural selection had two strings to her bow in the maintenance of type: a greater incidence of mortality among ordinary variants is the general method, and with extra forms there is usually protective sexual exclusion.
What would be the effect on the type of a species if it was partially protected from natural selection? It is self-evident that in such circumstances any variant, whether it was a progressive modification or a reversion to ancient type, would have a chance of existence more nearly equal to that of the typical individual than it would under conditions of severe selection. The atavistic revenant and the progressive variant would be protected against the hostile conditions that would otherwise make their destruction certain. The variation curve would become irregular. It would no longer be the same as under the more usual natural conditions. The members of the species falling within the absolute mean would become relatively less and a portion of the individuals that would fall under the average in a species subject to selection would now have a tendency to be distributed progressively towards the ends of the curve. Under protection, therefore, variation would tend to have an increasing influence on type.
That this is so among such highly protected species as the domestic animals is quite evident. If we take one of the latest additions to the category, the African guinea-fowl, where a comparatively short period in protection has created the common domestic tendency towards albinism, it will be found that there is no selective tendency adverse to the patchy individuals other than that brought about by man's intervention. It is interesting to note the effect of variation on hereditary type in this species. If a number of domestic birds are allowed to mix with wild birds under natural conditions, white marks and patches will appear for perhaps three or four generations. Then they entirely disappear.19 If, on the other hand, they are mixed with wild birds in captivity, or recently tamed ones, the hereditary effects on type are far more lasting. I know of cases where a very slight admixture of domestic blood has caused the persistence of white patches for twenty years after its introduction. It is hardly possible to doubt that in the wild flock the variation is eliminated by natural selection, and that it continues in the protected flock because of the weakening of selection.
One would anticipate that the chacma, just to the extent that it is protected
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by its ‘intelligence’ from the more rigorous natural selection, would show a tendency towards the same variation that is conspicuous in man and the domestic animals. And such anticipation would be verified by a very little observation.
When we commenced our investigation we had no such anticipation, and yet one of the first impressions we gathered was that there was something very much wrong with the type of the chacma. It was possible in a comparatively short time to learn to know and to recognise individuals in a large troop, and this was contrary to all our experience with gregarious mammals outside the order of the primates. It took perhaps a little more time and a little closer study than would be necessary to know and to distinguish the same number of individuals among an unfamiliar human race, but I do not think that the closest study or unlimited time would enable one to attain a like result with the same number of springboks, for instance. The first explanation that suggested itself was this: that, since the chacma, after all, more closely resembles the human being than any non-primate mammal, an eye trained to know and to recognise human individuals would discriminate more easily in this closely related species than in a more distant one. But more exact methods of observation soon convinced us that the ultimate reason lay in this and in nothing else: that there was a greater and more extreme individual divergence from type in the chacma than in any ‘natural’ non-primate species known to us.
There was a greater difficulty when one approached the question of definition. What exactly was this variation? It was at once apparent that certain conspicuous modifications had a tendency to recur constantly, and it was perhaps possible to ascertain the extent and the percentage of their occurrence. But these outstanding ‘single’ modifications did not lie at the root of the matter. It was rather the sum of an infinite number of small variations in each individual that rendered recognition easy. We ascertained the existence of certain distinct correlations, such as, for instance, the shape and size of the occipital ridge to the relative length of the arm, and exhaustive research would no doubt prove that such correlations of growth are attendant upon every somatic modification, great or small. But generally our own attempts at systematising yielded negative results, and gradually the conviction was forced on us that variation in the chacma is so irregular as to defy definition and to baffle all effort at classification.
I think just the same difficulty would be experienced, to an enhanced degree, in attempting to classify or define divergence from type in man. It is possible that all these variations may occur in accordance with the Mendelian laws of heredity, but even this we were unable to ascertain either experimentally or by observation under natural conditions. Though one thing seems certain: this variation is of the same nature as that occurring in man and the domestic ani- | |
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mals. These species are, of course, far more completely protected from natural selection than the chacma, and the tendency in them is therefore far more advanced. In man, the most highly protected of mammals, individual divergence from type has reached an extreme point. No single organ or function of his body is exempt; even the vital organs show a degree of variation for which identical conditions in nature will be sought in vain.
It seems, therefore, a safe assumption that the tendency to vary in the primates must be progressive, and that the rate of progress is determined by mental development, even if most of the modifications are in the nature of acquired characteristics created by the environment only, and are therefore not transmissible. Some in each generation would be hereditary and must have an increasingly adverse influence on the maintenance of type. Psychologically, the same extraordinary divergence is apparent in the chacma. Reference has been made to the difference between the vervet monkey and the chacma as far as hereditary individual memory is concerned, and the fact was noted that in the chacma there was considerable variation in the inheritance of phyletic memory, which in the lower species was quite uniform.
This psychological variation, like somatic divergence, seems to be reducible to a more or less regular curve. At the one extremity come the individuals with highly developed phyletic knowledge and proportionately undeveloped ‘reasoning’ powers. Their mind is instinctive, their reasoning ‘animal-like’. The casual observer would describe them as ‘stupid’. In the middle comes the common type in which the two souls are equally developed. These individuals inherit a certain amount of hereditary memory - less than the lower type does, but still certainly directive - and their individual causal memory is also ‘average’. At the other extreme come the highly intelligent animals - the clever ones - who are nearly devoid of hereditary memory. And between these three types come the usual gradations.
One would, on anthropomorphic grounds, be inclined to assume that the highly intelligent type of mentality would be the one most likely to be beneficial to the species in the struggle to live, and that this must therefore be the one towards which natural selection tends. It would appear, however, that as far as the chacma is concerned, both the ‘animal’ soul on the one hand and the more human-like soul on the other are, under existing conditions, in the nature of less advantageous variants. The mentality combining both types more or less equally is clearly the one which has thus far proved most beneficial to the species. But it is quite conceivable that environmental conditions might arise under which the higher mentality would be selected and a new variety or species thus created. That is a conception more in accord with our present knowledge of evolution than the popular erroneous conclusion that natural selection must necessarily be working towards the establishment of ‘human’
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mentality in all primates. There would be just as much reason for assuming that the long-necked okapi (Okapijohnstoni) must necessarily in time become a giraffe.
One can hardly fail to notice the analogy which exists between the psychological variation curve of the chacma and that of man. Human mental variation has at the one extreme congenital idiocy and all the grades of ‘weakmindedness’ up to the average intelligence. On the other side come the mental ‘prodigies’, the artists, the poets, the geniuses and, finally, the insane. These extreme variants seem to be representative of the same phyletic types of mentality that are conspicuous in the chacma.
It would seem that in the chacma mental variation is very closely correlated to the general shape of the skull. The two extreme cranial types are, at one end of the scale, the pronounced cynocephalic form with excessive orbital ridges, a greatly developed occipital ridge and an extreme orbital structure,20 and at the other the ‘round-headed’ type in which these characteristics are much less developed and the general shape of the skull tends towards that of the anthropoid. Needless to say, the more human-like mentality is invariably associated with the more anthropoidal cranium, and the animal-like with the cynocephalic.
It is interesting to note that this cranial variation of the chacma is ‘recapitulated’ in ontogenetic development, and from the nature of the development of the individual one would infer that the chacma is descended from an ancestral form which far more closely resembled the existing anthropoids and which may have possessed a higher mentality than the existing type. The chacma foetus and a newborn baby are far more chimpanzee-like than the adult is. Indeed, at certain stages of development the foetus resembles the adult chimpanzee more closely than the adult chacma. Even the first hair is quite black and resembles that of the chimpanzee in growth and texture. If a series of skulls, from foetal to full maturity, are compared it will be seen at once how the cynocephalic characteristics increase with age, and what a great difference there is in this respect between the foetal skull and that of the adult.21 22
It is, I think, the persistence of the anthropomorphic type of skull during youth which has given rise to the popular idea in South Africa that there are two species of baboons - long-faced and short-faced - and that the short-faced are always ‘cleverer’. The ‘short-faced’ variation certainly does occur in adult forms, and, as I have suggested, it is associated with a higher type of mentality, but it does not occur to such an extent in mature individuals that it could have suggested the idea of two distinct varieties. I think the only explanation is that young ones are mistaken for individuals of a distinct variety and the occurrence of an occasional adult ‘short-face’ served to strengthen the impression. There are better grounds for believing that occasionally either the ‘long- | |
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faced’ or ‘short-faced’ variation has shown a tendency to become predominant in certain isolated troops; our own observation, however, afforded no evidence at first hand of the existence of such a tendency and I think second-hand evidence in its favour should not be accepted without reserve.
On even more dependable evidence rests the assertion that many years ago in an isolated troop in Rooiberg the tendency to albinism appeared as a hereditary characteristic. We saw a white female that was said to have been captured from this troop as a baby. A trustworthy informant, a Wesleyan missionary who had been stationed for many years in this country, informed us that at one time he estimated that ten per cent of the individuals in the troop showed white bands and patches. Pure white ones were rare. If the characteristics were really hereditary and not due to the conditions of the environment or to something in the nature of an infectious disease, the circumstance would be extremely interesting, pointing to the origin of a genuine mutation. What makes it doubtful is the fact that although true albinism is quite common among natives, its heredity in the human being is highly questionable.
One would anticipate that the anthropoids, possessing as they do a far higher protective mentality, would exhibit generally greater individual divergence from type than the chacma; and our present somewhat scanty knowledge of the great man-like apes seems to confirm this assumption.23 My own knowledge of the chacma leads me to believe that some of the different forms of the chimpanzee which have in recent years been discovered in this country and which have been assigned to distinct varieties will eventually be found to be continuous variants of the same species. Even now it is becoming difficult to draw a hardand-fast line between two such widely different forms as the gorilla and the chimpanzee. Several specimens of gorilla-like chimpanzees and chimpanzeelike gorillas have been secured which point to the probability that those two forms are connected by an unbroken chain of intermediate types. It is possible, as I have pointed out in the case of the chacma, that there may be a strong tendency in the higher primates for certain variations to become predominant in isolated groups. That is certainly so in man. It is impossible to imagine how else the different races of human beings could have originated. Now I am inclined to think that an analogous thing has happened or is happening both with the African anthropoids and with the orang-utang - that variations which were ‘continuous’ are apt to become predominant in groups through isolation and that to this process in nature are to be ascribed the different local ‘races’ of the big apes.
It is hardly necessary to point out that, if this proved to be true, there is no similar persistent process discernible anywhere in nature in species subject to rigorous natural selection. Among lower animals, where a distinct variety has been established by natural selection, it is generally kept within its distinct
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limits by those insurmountable barriers of sexual selection and the sterility of hybrid offspring. I am strongly inclined to think that the offspring of no two sub-races of the same anthropoid will be found to be sterile.24
As far as the chacma is concerned, however, there are none of the elements of uncertainty which render the formulations of all such hypotheses with reference to the anthropoids merely tentative suggestions. Our knowledge of this species is wider and more definite, and it can therefore be stated without reservation (a) that there is greater individual divergence from specific type than in any other non-primate mammal in this country, and (b) that the only possible cause of this divergence is the same as that which creates a similar tendency in the domestic animals and man - namely, the ebbing force of natural selection.
It must not be thought that this variability due to protection is confined to the primates and domestic animals. It is a universal law of nature (and its methods of operation have been indicated in this chapter) that variability increases as the struggle for existence becomes less severe. Under natural conditions there is perhaps no better instance of protection from selection than that afforded by oceanic islands. The isolation and consequent want of immigratory competition, the absence of natural enemies, an equable climate and a plentiful food supply constitute for many species a highly protective environment in such islands. And the result is always the same: extreme variability. And if these varieties are localised, they are often in surprisingly small contiguous and confined groups.
Most naturalists who have examined the living forms in such islands have been struck by these circumstances. In the Sandwich Islands, for instance, there are over three hundred recorded species of the land mollusc Achatinella. Every valley has a localised peculiar variety. So too in the Celebes - Fritz Sarasin has described the astonishing variation of the same genus, united by an unbroken chain of intermediate forms; and the same thing occurs in higher forms in a great many remote islands. But although this fact has been observed frequently, the natural law of which it is the result has never been clearly recognised and defined as far as I know - the law that protection from rigorous selection means a smaller death-rate among the variants of a species. |
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