The History of Biological Theories

Claud Bernard had asserted that we must look to the organism itself, and not to any force outside it, if we wish to discover the final cause of any physiological process ; for every living organism is an end unto itself, and is subject to the inner laws of its own being. Gaston Bonnier (1889) attacked the theory from this point of view. He claimed that the size and colour of flowers bears no definite relationship to the number of insect visitors. Mignonette, for example, has many visitors, while large lilies have very few. There are many lovely and sweet-scented flowers which have nothing to offer a visiting insect. Bonnier, like Miiller, gave many facts which seemed to support his views, but this attempt to refute Darwin's theory made no impression.

Darwin's ideas about beauty were not always accepted even by those who believed in evolution. The weak points in his theory were, perhaps, nowhere so obvious as in his attitude toward sexual selection. He needed instances to show that males who are not beautiful are rejected by the females they desire, and that the progeny of weakly and less attractive males does increase less quickly. But he did not attempt to find these. He did not even prove that there is a definite choice by the female, still less that such a choice, if it exists, exerts any influence upon the number or nature of the offspring. For this reason Wallace rejected the theory of sexual selection altogether. He believed that the more intense colouring of the male is due to his greater energy. It is, he held, a general rule that thin and sickly animals are dull of colour, while healthy ones are brighter of hue. He thought that natural selection plays a part here, for the stronger male will win a mate more easily, and, being strong, he will also be beautiful.

Wallace also drew attention to the beauty of certain fruits black elderberries, blue sloes and bilberries, white mistletoe-berries. The colour of these fruits attracts birds, he suggested. The birds eat the soft parts of the fruits and scatter the seeds. Other fruits, certain nuts, for example, are green, and so are hidden from the eyes of birds ; some, like the chestnut, have a further protection in the form of prickles. As a rule Darwin and his followers simply discussed the causes which had led to the differences between male and female. They did not delve any deeper into these problems of sex. Such differences are, in the first place, differences in the constitution of the sex glands and the actual reproductive organs, and, secondly, differences in coloration, marking, and size of body, as well as difference in instincts. The first group include the so-called 'primary sexual characters', while the second include all the 'secondary' characters. When Darwin discusses animal beauty he deals almost entirely with these secondary sexual characters ; some biologists, however, believe that these are entirely due to the influence of the primary sex glands. They think that the development of the colour, markings, horns, antlers, and other characteristic 'mascu-

line' adornments are due to secretions of the male glands, while these glands also inhibit the development of the corresponding feminine qualities. The female glands have exactly the opposite effect. These workers point out that castrated animals often exhibit certain characters which are usually only found in the opposite sex. Some investi- gators have held that the male is a more highly developed (advanced) form than the female (Jager and Eimer have called this 'male prepotence'). Others have favoured the opposite view that the female is a degenerate male. This is a development of the old idea, due to Aristotle, that the female is the embodiment of matter, the male the embodi- ment of form. The female, they asserted, was originally at least as beautiful as the male ; but it expended so much of its energy in providing nutriment for its eggs and in caring for its young that it gradually degenerated.

There were others who believed that the striking and beautiful male represents a sort of sacrificial offering in- tended by Nature for the enemies of its kind ; in this way the less conspicuous female would be protected, and so the increase of the species would be secured. All this will serve to illustrate the way in which men speculated upon the beauty of plants and animals. It is plain that both Darwin and certain of his opponents thought of natural beauty as a homely countryman might think of it bright colours, crude and glaring decorations, horns, antlers, long feathers, crests, &c., &c. in short, all the unusual and striking features of the natural world. All their theories refer to details of this kind and they pay no attention to that truer and more subtle beauty which reveals itself in fine though inconspicuous markings and shadings, in beauty of line, in harmony and in contrast of colour, in movement, in song, &c., &c.

Those who opposed the Darwinian theory recognized to the full the weakness of these attempts to derive the beautiful from the merely useful. The Duke of Argyll attempted to show that there are definite aesthetic laws underlying many of the appearances in the realm of nature, and that these beauties are not there simply because of their usefulness. Every now and again some investigator would affirm that beauty has an objective value. Brunner von Wattenwyl, a Viennese entomologist, examined the coloration and markings of certain beetles, and declared that the whole arrangement, with its definite lines, circles, and the like, follows definite aesthetic laws, and is not determined by morphological ones only (1873). Suppose the markings were really determined by morpho- logical characters, the thorax of the beetle would be differently marked from the wing-covers, since these two structures are not of the same morphological nature. Some beetles, however, are marked in circles, one half of each such circle being on the thorax, and the other on the wing case (elytra). This arrangement can only be explained by reference to some aesthetic law.

Brunner quotes other examples of the same phenomenon, and asserts that every organism embodies a principle of perfection, and that the beauty of the organism is merely an expression of this principle. These views, expressed by Brunner, were not very sympathetically received; nor was the work of Hallier, the botanist, on The Aesthetic Motive in Nature (1890). This book is lengthy and tedious, but in it he points out certain facts that had hitherto escaped observation; that plants, for example, usually have a pleasant smell, while animals, as a rule, have an unpleasant one; that the foliage of plants bearing blue flowers is never a full green, but always a bluish or yellowish green ; that we can deduce certain definite aesthetic laws from a study of the distribution of colour in Nature, &c., &c.

The colours of flowers do not merge into one another ; there are two definite series ; one, the xanthophyll series, range from white through yellow and orange to red ; the other, the anthocyanin series, pass from white, through pale blue and deep blue, to violet. Related forms have colours belonging to one or other of these series, to which our attention was first called by de Candolle. Very little work has been done on this subject, and it would be worth while to examine this classification more closely.

Those who have studied beauty from the aesthetic point of view have not protested against this attitude of the scientists, which banishes all belief in objective beauty in Nature. They promulgate certain generalizations about beauty in Nature, and then proceed to discuss beauty in relation to the works of man that beauty which man produces in imitation of Nature. If we attempted to gain some idea of Darwin's real attitude to Nature from his theory of the beautiful, we should be tempted to believe that he had no real sense of natural beauty. We should be quite wrong in this conclusion, however. His theories about beauty may seem to us to be superficial, but his interest in the beauty of Nature is always delicate and human :

*I used to like to hear him admire the beauty of a flower ; it was a kind of gratitude to the flower itself and a personal love for its delicate form and colour. 'I seem to remember him gently touching a flower he delighted in ; it was the same simple admiration that a child might have.' l This is how his son pictures him for us. He tells us that Darwin, when quite an old man, still kept a keen look-out for birds' nests, that 'he sometimes showed a tendency to strong expressions'. He describes, in the Origin, a larval cirripede 'with six pairs of beautifully constructed natatory legs, a pair of magnificent compound eyes, and extremely complex antennae'. 'We used to laugh at him for this sentence,' his son tells us, 'which we compared to an advertisement' (Life and Letters).

HERE are many ways in which living organisms may resemble one another. When alike in bodily structure we say they are related and call their similar parts homo- logous. Where parts are merely similar in function we say they are analogous. /'Lastly, organisms may be similar in habit, thus the lime and chestnut, the fir and larch, are of similar habit, while the bushy poplars have a habit quite unlike that of the pyramidal poplars. It is not customary to speak of similarities of habit among animals, but examples may be found among them too. Thus the 'swallow-taiP butterfly is much like the humming-bird in appearance and in its characteristic flight, while the habit of the shrew is like that of the mouse, that of the ostrich like that of the giraffe.

Such resemblances may, however, be only accidental. We may, then, ignore them, and define homologies as resemblances which affect the fundamental structure, while analogies are more superficial likenesses which are, so to speak, superimposed upon the homologies. Yet the effect of analogies may be far-reaching too. The fin of the whale, which is analogous to that of the fish, is merely an external modification of the mammalian limb, and its internal structure is determined by the fact that the whale is a mammal.

Other resemblances occur which are even less intimately connected with the nature of the organism. They do not concern the plan of the body, nor are they dependent upon the functions of its various parts, but relate to such apparently unimportant details as the external shape, coloration and markings, to certain bodily habits, and the like. "^Animals and plants which have very different morphological characters may be similar in appearance, even in the smells or the sounds they emit, and it is diffi-

cult to ascribe these similarities to mere chance. We say then that they are 'imitative', and the phenomenon is called 'mimicry's/There has been no attempt to analyse the widely divergent phenomena included under this head, no attempt to deduce the laws of mimicry. Hence we must fall back upon a purely empirical classification, which merely serves to illustrate how varied these devices may be. 1. Two species of the same order may be 'imitatively' alike in their bodily workings the butterflies belonging to the Leptalideae resemble those of the Heliconideae in colour, marking, and shape of body." x *The two snakes Python amethystinus and Dipsadomorpbus irregularis belong to widely divergent groups, and yet are almost identical in appearance.

2. Animals belonging to different orders may resemble each other: certain butterflies (Sesia crabroniformis) and certain wasps (Fespa crabro) for example. In the fly Volucella bombylans two varieties are distinguished. The larvae of this fly are parasitic on humble bees. In one type the larva lives on Bombus muscorum, and the mature animal is like this bee, while the other resembles Bombus lapidariuSj in whose cells it is nursed through its larval stage. The wasp Mygnimia aviculus is very much like the beetle Coloborrbombus fasciatipennis, &c.

3. Animals, in their marking, may simulate their surroundings. The leopard and the jaguar have black eye- like spots on a yellow skin. Lubbock suggested that these give the dappled effect of the splashes of sunlight which fall on to the tree-trunks through a canopy of foliage. The tiger's stripes resemble the bars of light and shade among tall jungle grasses. Insects show this sort of mimicry particularly well. Some dragon-fly larvae cover their bodies with slime, and so cannot be seen against the back- ground of the pond bottom where they lie in wait for their prey. Some night-flying butterflies, which lie con- cealed among lichens, are coloured greenish- white like the lichens. Many more examples of the same type of mimicry might be given.

4. Some animals do not mimic their surroundings, but exhibit sympathetic coloration; thus the lark, which is grey like a clod of earth ; or the grass-green grasshopper. Arctic creatures are apt to be white. They may retain this coloration throughout life, or they may change it with the seasons. Other animals can alter their coloration, suiting it to the colour of the environment. The pupae of certain butterflies (Pieris) are differently coloured according to their surroundings. An analogous pheno- menon has been observed among certain lizards, which after each moult adapt their colour to their habitat, but when the skin has once hardened the colour is fixed. The tree frogs can alter the intensity of their green colora- tion to tone with that of the surrounding foliage. Chame- leons and many sea animals change their colour with the environment (cephalopoda, crustaceans). The lack of colour seen in many sea and freshwater animals renders them almost invisible in water.

Among birds this imitative resemblance is also shown in their eggs, which often resemble their ordinary sur- roundings. Eggs laid in darkness those of the wood- pecker, for example are white, while those laid in open nests are speckled green or brown. Certain birds which lay in sand have sand-coloured eggs. 5. Animals have instincts which enable them to in- crease this resemblance to the environment. The par- tridge hugs the earth. The green grasshopper chirps from a green bower. The sharks lay brown four-cornered eggs, the corners running out into four long thin bands ; these eggs mimic the seaweeds on which the shark suspends them.

Some investigators regard simulation of death by certain animals as mimicry; that animals which visit flowers often resemble flowers in colouring is probably a related pheno- menon ; butterflies, blossom-inhabiting beetles, humming- birds are among the most beautiful of animals. 6. Mimicry is exhibited not alone in form and colour but also in smells. There are plants which emit a corpse- like odour by which they attract flies, and beetles which smell like roses or like musk, and there are many other well attested cases.

7. There is also mimicry in the world of sound; the yellow-hammer, particularly certain steppe varieties, chir- rups like a grasshopper, and some lizards emit similar sounds. The song of one bird may be like that of another. 8. Plants exhibit mimicry as well as animals. The alga Caulerpa has organs which recall the root, stem, and leaf of the higher plant. Some plants have branches which look like leaves. The inflorescence of the Compositeae is like a simple flower. A species of Mesembryanthemum, which grows on the South African Steppe, has leaves closely pressed down on to the earth, which resemble stones. The flowers of orchids may imitate flies, bees, spiders, &c.

It has not been determined how common mimicry is, nor how it is distributed in the plant or animal series. It is not entirely unknown even among the lowest forms of life, as the Protozoa and Coelenterata ; but it is commonest among the most mobile animals, especially the Arthropods and the Vertebrates. We have shortly enumerated the chief types of mimicry, using that word in its widest sense. Not all the cases alluded to would be regarded by Darwin- ists as examples of mimicry. They look at the matter from the utilitarian point of view, and consider only those devices mimetic which are useful to the animal in its struggle for existence.

This view has a history. Darwin's grandfather had already pointed out that some animals are able to hide from their enemies because of their colouring (Erasmus Darwin, Zoonomia, 1794). This phenomenon was often referred to, and in 1862 was studied more thoroughly by Bates. He noticed that certain butterflies of different orders are strikingly similar, and he assumed that these similarities were only to be explained by the Darwinian theory. One of the butterflies has a nasty taste, but the other is palatable to birds. Since, however, the palatable resembles the unpalatable form, the birds do not pursue

it. In the view of Bates ('Contributions on the Insect Fauna of the Amazon Valley', Trans. Linn. Society, xxiii, p. 495) the two butterflies chanced originally to be some- what similar in appearance. Birds could distinguish between them and hunted the edible forms. Among these there would be some which were particularly like the inedible ones; these would be less preyed upon by the birds, and so would tend to survive. From among their offspring those most closely resembling the unpleasant- tasting butterfly would again survive, and so in the course of ages the similarity seen to-day has arisen.

Bates gave the name of mimicry to this type of resem- blance. Darwin accepted the observations and explana- tions of Bates, and mimicry became one of the chief arguments for the evolutionary theory. Other possible explanations of mimicry were ignored. The suggestion made by M. Wagner (1880), for example, that animals are forced to choose environments which they in some measure resemble, by their desire to maintain themselves alive : or the hypothesis due to Cope and Brun- ner that the qualities of animals, like those of the chemical elements, are not infinite in number.

The Darwinians thought that protective mimicry should be distinguished from aggressive mimicry; the former helps the animal to hide from his enemy, the latter enables him to make himself invisible to his prey. Wallace would also distinguish between two further types of coloration and marking. Firstly, there is warning colora- tion, which is hardly mimicry; indeed, it is the anti- thesis of it, as in the brilliant colours of inedible cater- pillars. Secondly, there are the distinguishing marks by which animals are supposed to recognize one another. Wallace suggested, perhaps rather fancifully, that the white spot on the rabbit's upward-curving tail serves as a signal to the young when the mother, scenting danger, flies to her burrow. The white spot on the tail marks out for the young the line of retreat.

important as they did in Darwin's time. The German zoologist Eimer (1889) and the Dutch lawyer Piepers (1903) early assumed a sceptical attitude. They pointed out that the chasing of butterflies by birds is not as every- day a phenomenon as the theory would suggest. Piepers lived for twenty-eight years on the Island of Sunda, and states that only on four occasions during the whole of his sojourn there did he see a bird chasing a butterfly. Other naturalists have expressed similar doubts, though there are some who have collected facts which seem to support the opposite conclusion. While this uncertainty exists as to how far there is any hunting of butterflies by birds, it is obvious that inferences concerning mimicry which depend upon an alleged chase can by no means be regarded as established.

There are other cases, e. g. the case of the two similar snakes referred to above, where the mimicry cannot possibly be protective, since both forms are equally liable to be attacked by enemies. Piepers also doubts whether the butterflies which were assumed to be inedible by birds are really so unpalatable. Moreover, he cites many cases of very close resemblance between butterflies which inhabit quite different areas. Some butterflies from the Cameroons are extremely like certain European forms, some from Natal like others from Siberia, and so on. But butterflies are, as Eimer points out, closely related to each other. Their bodily organizations are similar, and the markings develop in accordance with that organization. Hence these resemblances have, according to him, arisen by chance.

Animals may repeat the colour of their surroundings in their skin colouring, they may 'photograph' it, in short. Eimer held this to be the explanation of many phenomena called mimicry. Piepers, in agreeing, further expressed the view that mimicry may be due to some suggestive in- fluence exerted upon the organism by its surroundings, may be a psychological phenomenon, in short. revolution in our ideas about mimicry. Formerly too much attention was devoted to the subject, now it receives too little. The phenomena are so numerous and occur in such manifold and striking forms that we cannot believe they are all due to mere chance resemblances. To study mimicry we need not go far afield, for the phenomenon is common enough in Europe, especially among insects.

To the historian, the modern point of view about mimicry is of great interest. We do not doubt the existence of homologies and of analogies. These ideas arose from a full examination of the facts. They come to us from pre-Darwinian days, and would remain even if Darwinism were abandoned. The ideas about mimicry, on the other hand, were introduced into biology as the result of the evolutionary theory. Darwin and his followers did not endeavour to ascertain what mimicry really is they were opposed to any abstract analysis of the phenomena of mimicry. They picked out striking, but isolated, examples, and then propounded explanations to which they attached great weight.

'Tp HE belief in progress in a humble past, a better JL present, a still more glorious future has become so much a part of our mentality that it is difficult to imagine a time when this thought was unknown. And yet Rousseau earnestly sought for facts to combat the idea that classical culture was higher than our own. On the threshold of the eighteenth century we find Fontinelle (1708) pursuing the same quest. Only the late eighteenth and early nineteenth centuries brought the conviction of continual progress. Herder was among the pioneers of this movement. HegePs influence was even greater. He taught that in the world of events there is progress from unconscious beginnings to conscious understanding, and that this development is the most important subject of philosophical study. In France, Condorcet and Comte were responsible for the spread of the belief that the human mind progresses : it begins with theological conceptions, and passes on through meta- physics to science. In England, two years before the Origin of Species appeared, Buckle had analysed the pro- gress of civilization. From Cuvier and Lamarck onwards biologists had been depicting a world which was originally inhabited by the simpler forms of life, and describing how these grew more complex, step by step, until Man was reached. Agassiz, in particular, wrote much on this theme. So too von Baer, in his work on embryology, re- ferred to the progress revealed by the study of develop- ment, and showed how the gulf between the various types widens as development proceeds. Milne-Edwards ex- pressed the same idea in a somewhat different form ; he believed that progress meant that, in the living hierarchy which is a body, there is an ever-increasing division of labour, both structurally and functionally.