The History of Biological Theories

The difference between Naudin and Godron is, how- ever, only a difference in the use of the words Species' and Variety'. Naudin assumes that there is evolution of new forms, as does Lamarck; but in practice he is fully con- vinced of the constancy of those characters which define the species, and he believes that new forms represent some new combination of such characters. Such new com- binations are due to crossing, and they do not mean that there has been any fusion of the specific characters, only a resorting of the same.

The French experimental work was for a time super- seded by Darwinism. Darwin and his followers looked upon such work merely as an attempt to find experimental support for Cuvier's definition of a species, and Cuvier had said that a cross between two different species could not produce fertile offspring. There was, however, to be a return to his ideas later. Among evolutionary theorists the idea that new species arise by saltations from previously existing ones was con- tinually being discussed. Bateson, in the introduction to his Materials for the Study of Varia ^1^(1894), has shown us how this hypothesis developed out of Darwin's views. He describes how enthusiastically Darwin's ideas were ac- cepted, how constantly biologists discussed these questions of heredity and of adaptation, how work on anatomy or embryology was always directed to the finding of new facts which would support Darwin's theory, and how indiscriminate the enthusiasm aroused by the theory was.

Bateson wrote to free men of science from this obsession. Although he did not know it, his thought is directly con- nected with those earlier ideas of the nature of a species which were rudely interrupted by the advent of Darwin- ism. He abandoned Darwin's main thesis, that there are connecting links between all species, and taught once more that there are definite gaps in nature's vast diversity of forms. The environment is too uniform to have been the agent which produced this great variety. New species originate suddenly and the sudden appearance of new characters is a much less rare phenomenon than is com- monly supposed. Bateson's work is chiefly concerned with the collecting and classifying of these 'discontinuous variations', as he calls them.

Bateson had no intention of challenging Darwin's funda- mental assertions. He did not realize that his point of view was directly opposed to that of Darwin, and that his theories represent a return to the earlier belief, that species are something more than 'aggregates of conveni- ence' of similar forms. Later workers who followed similar lines of investigation were also quite unconscious of this antagonism. Kor- schinsky's botanical work resembled that of Bateson in zoology. In 1899 and again in 1901 he pointed out how often, in practical gardening, new forms suddenly arise and then remain constant. The common Acacia has given rise to the following new varieties quite suddenly and without any obvious reason. In 1833 a form without thorns; in 1855 one with simple leaves; in 1862 one with pink blossoms, in 1862 also a form which is a very profuse and early bloomer. The double petunia arose in 1853; Cyclamen persicum has been grown as a garden plant since 1731, the double form only appeared, however, in 1850.

From these, and many other similar instances, Kor- schinsky deduced that species have not arisen by any gradual process such as Darwin believed in, but by occa- sional sudden changes; he called this method of pro- duction 'heterogenesis'. In all other respects his views agreed with those of Nageli, and he believed in a principle of perfection. De Vries investigated the subject of discontinuous variations, or 'mutations'. While Bateson and Korschin- sky took their data from books, De Vries attacked the problem experimentally. In his fundamental ideas he was in agreement with the French school with the work of Jordan on the one hand, and of Nageli on the other. He drew a sharp dividing line between artificially produced species in which fluctuating variations are accumulated by artificial selection and natural species which arise by mutation. The amount of sugar in the beetroot, the juiciness of the carrot root, are examples of fluctuating variations ; these have been increased by cultivation, and are passed on by heredity, but disappear directly the plant reverts to wildness. Mutations deviations from the parent species which appear suddenly and without any obvious cause are quite different in their behaviour. They 'breed true', and hand on their special peculiarities from generation to generation.

In the year 1886 De Vries began to study the mutations of the American plant Oenothera lamarckiana (Evening Primrose). He obtained a whole series of mutants from the parent form, e.g. O. gigas, which has broader leaves and sepals, the foliage leaves more curled, while the fruits are shorter and contain fewer but larger seeds than do the fruits of the parent form. These mutations were re- garded by De Vries as constituting 'elementary species', and, according to him, such mutations or saltations represent the first steps in the formation of new species. The struggle for existence destroys those that are valueless, while those that survive give rise to fresh mutations, and so the variety of living forms has been produced.

Many Darwinists accepted De Vries' theory, but it has also been subjected to considerable criticism. De Vries observed that the same mutant may arise in different ways not from the parent form only, but also from other mutants. His critics pointed out that these changes may simply be developments of latent characters, and not advances towards a new type. Though with less justification perhaps, there were others who did not admit that there is any really fundamental difference between fluctuating variations and mutations. There were philosophical objections as well. Driesch and Whitmann have rightly pointed out that we cannot re- concile our ideas of a reign of universal law in the organic world, with this assumption that spontaneous and purpose- less mutations may occur.

Some workers tried to reconcile the views of De Vries and Darwin ; Keller for example, suggested that our various races of domestic animals have sometimes arisen by the selection of fluctations (e.g. the Simmenthal cattle), some- times by sudden mutations (Dachshund). The true Dar- winian point of view however even his view of evolution bore only a very superficial resemblance to the views of De Vries ; the two theories are really fundamentally different.

Beginning with a discussion of the idea of 'elementary species', the analysis of this idea of 'species' has been carried still further. The individuals of each elementary species and of each mutant are not all alike. If two individuals pair, their offspring will have some of the paternal and some of the maternal characters ; and since the parents are not identical, their characters will be differently combined in their children. Some plants and animals are self-fertilized. In these the characters of the individual remain constant throughout any one series which is the result of self-fertilization.

Johannsen (1903, 1909) succeeded in separating two such series in a bean (Pbaseolus vulgaris 'princesse') which is always self-pollinated. He divided the seeds produced by the plants of one bed into three groups, according to their size, and planted each group separately. He divided the seeds of the second generation in the same manner. After several generations he obtained from the small seeds plants which only bore small seeds, from the large seeds

plants which bore large seeds only. The original bed must have contained a mixture of forms ; if we call this group of forms a 'population', as Johannsen did, they must have been the product of at least two ancestral forms, as the above experiments show. Johannsen called these two ances- tral forms 'pure lines'. The beans were self-pollinated, and hence these two strains ran parallel to one another. Similar 'pure lines' exist in other forms, but owing to the necessary crossing of two individuals which sexual repro- duction entails they become obliterated. This idea of pure strains forms a strange conclusion to Darwin's theory of species. It is much more characteristic of the biological thought of the day than is the Darwinian conception of species as merely artificially determined groups.

De Candolle regarded a species as an intellectual abstrac- tion from some group of forms that happen to be under examination. Darwin, on the other hand, looked upon it as a tree in whose stem all branches flow together towards a common root. Johannsen suggested a theory which linked up the formal and the genetic ideas. A species consists of a stream of similar forms which run a parallel course in evolution. The differences between some species are so minute that they are often less obvious than are chance fluctuations due to the action of the environment. Quetelet and Francis Galton introduced the statistical method into biology, in order to find the middle value in a series of varying forms. This now forms an important branch of applied mathematics, into the details of which we shall not enter ; we will taerely illustrate its use by a few selected examples.

The beans produced by any garden bed differ in size the differences being due to inner and also to chance external causes. If we separate these beans into groups according to their length, and arrange them in a series of glass tubes so that the longest are placed in the first tube, the next sized in the second, and so on, the upper surfaces of these bean columns will form a so-called 'Gaussian curve of error', or normal curve. Beans of medium size will be the most numerous, the largest and smallest sizes will be the rarest ; from the maximum point the heights of the columns will diminish symmetrically towards both ends of the curve.

From this we deduce that there is an average value which all beans tend to attain. External factors, however, cause some to become larger than the average, others smaller, and, since the variation curve is symmetrical, we conclude that the factors making for largeness are, on the whole, equal in potency to those making for smallness. Some cases are more complex. We might, instead of the size of beans, consider the number of axes in an umbel this number is also variable. From among a large number of plants we would group together those with six, those with seven axes, and so on, and we should find, as before, that there is an optimum number which occurs in the greatest number of specimens. The final result might, however, be different from that obtained from the study of beans.

Ludwig obtained the following results from studying the axes of the first order in the umbel of Torilisanthriscus: The maximum value is not the middle value but there are rather two maxima, namely at eight axes and at ten. The reason for this was that here two forms were growing together which differed from each other in the average number of axes. Ludwig eventually found these forms growing separately in other districts, and when he in- vestigated the number of axes statistically he obtained the following results. In one place

This example illustrates the importance of the statistical study of variation. No analysis of the characters of one or of a few individuals gives us a complete and comprehensive view of the nature of any type. A statistical examination of a large number of forms will render obvious the existence of certain boundaries which other methods do not reveal. Following Johannsen, species and varieties which have been statistically defined are called Phaenotypes, for their properties are determined by their external features only, and no assumption is made as to whether these features are inherited or no. Johannsen distinguishes between these and Genes such elements, tendencies, and con- ditions as are determined by the reproductive cells. It is suggested that each reproductive cell contains a series of genes, i.e. of the rudiments determining characters. The discovery of these genotypically determined charac- ters becomes the task of the Mendelian worker ; he uses Mendelian methods of crossing and of pure breeding; such discoveries are among the most widely discussed problems of modern experimental biology.

For a long time one criticism of Darwinism has been that there is no such gradual variation as he postulated things vary round a mean value. Statistical methods give new support to this assertion. It is possible, by artificial selection, to cause some change in this mean value, but the alteration cannot be carried beyond a certain point, and if the artificial selection ceases, the things revert to the original mean value. Finally, we may mention that problem so often referred to by the Darwinists Does variation occur within the same limits among both the higher and the lower forms ? This has, up to the present time, only been discussed in very general terms. Many investigators have assumed that the lowest forms, and the forms found in the oldest strata of the earth's crust, show more tendency to variation than

do the higher, more recently evolved types. They point out the greater complexity of the more highly organized types, and assume that this must restrict their variability within narrow limits. G. Seidlitz, Daniele Rosa, and others held this view. Until recent times there was a fairly general conviction that species are simply groups defined by anatomical characters, and that their physiological and biological pro- perties are the result of their structure. This view was held in spite of the fact that Lamarckianism, Vitalism, and the Darwinian theory were fundamentally opposed to any such assumption. Lamarck declared that the animal's own efforts lead to the formation of new organs ; the vitalists regarded function as higher than form; while it was the fundamental hypothesis of Darwin that the conditions of life determine the form of the organism. Yet such was the strength of the old anatomical ideas, that none sought to define a species by any other than morpho- logical characters.

Nevertheless organisms do show physiological specificity. When we speak of a blood-relationship between animals we no longer mean merely a genealogical relationship. Landois in his day carried out some interesting experi- ments, and these have been repeated more recently under different conditions by H. Friedental, G. Nut tall, and others. They were designed to prove that human blood is chemically more like the blood of the anthropoid ape than of any other animal.

These experiments were cited as affording a new proof of the Darwinian theory. Even the budding and grafting of trees, and of animals, where these processes can be successfully carried out, depends upon a near physiological relationship ; it is only possible between nearly related forms. From such experiments it should eventually be possible to discover physiological affinities and relation- ships between living organisms. Before this can be done,

however, the problem will require considerable investiga- tion, that we may see how far physiological affinity corre- sponds to anatomical relationship. The idea that organisms may be physiologically related leads us to the idea of physiological species. The well-known wheat rust (Puccinia graminis) exhibits in its life history a peculiar alternation of generations. In its first phase it is a parasite living on the leaves of barberry and forming spores (called aecidiospores). If these are carried to a grass they grow and produce patches of rust ; the reproductive bodies of these plants are summer spores (uredospores) ; these produce rust on other grasses ; finally, in autumn, winter spores (teleutospores) are formed.

J. Ericksson (1884) proved that in this rust fungus six species may be distinguished. These are anatomically alike and are only to be recognized physiologically. The one, the uredospores formed on the oat, for example, will only infect this grass and one or two other species ; it will not infect wheat, while the wheat rust can only germinate on wheat and not on oats. Ericksson called these species 'biological species' ; others have called them 'physiological species', 'species sorores', 'formae speciales', 'Gewohn- heitsrassen'.

Similar facts are known about parasitic animals. The worm Tylenchus scandens, when it had lived for several generations on grasses, lost the ability to live on bulbous plants. The aphis Chermes strobilobius is anatomically practically identical with Chermes lapponicus var. praecox ; the two can only be distinguished by their mode of life. These observations merely represent a tentative effort to prove that genera and species can and must be defined by their physiological, biological, and psychological char- acteristics, and not by their anatomical characters only. Agassiz had already put forward this suggestion, but his opponents had found it unacceptable.

We may close with a suggestion. The biological species Tylencbus scandens is defined by the fact that this worm has weaned itself from a diet of onions. No one has objected to the establishment of a new species on these grounds ! Yet what scorn would be poured on any investigator who should suggest that the morality, the religious sense, or the intelligence of man could be used as a basis of classifica- tion to divide mankind into separate classes !

7. Variability regarded as the Result of the Influence of Environment on the Organism. The question of the relation between the organism and its environment is not entirely modern. It has been one of the main themes of philosophy since the time of Locke and Leibniz. Hume and Kant began with the same problem in working out their systems, but they were concerned not with organisms in general, but with one special organ the soul of man as were the later philosophers. Leibniz propounded the thesis that the surroundings do not influence the soul ; it develops as the result of its innate tendencies, which are there from the beginning. It receives nothing from its environment, nor can it influence that environment. The surroundings have no power over the organism ; it runs its own course, and in it the surroundings are, as it were, mirrored, but they do not alter it in the slightest degree. Locke took the opposite view that the environment has the power to store up experiences in the soul, and so, in this way, to change the nature of that soul. Hume attached still greater importance to the influence of the outer world upon man. Like the modern theorists, who declare that the organism is nothing but an accumulation of sense- impressions, he looked upon the soul of man as a mere bundle of sensuous experiences. Kant sought to bridge the division between Leibniz and Hume. It is true that he recognized the significance of experience, but he held that the soul has the power to determine in what way its surroundings shall influence it. Modern biology is still quite unable to solve this problem; it sees on the one hand the active individual, which feels itself to be inde- pendent of the world around it ; on the other hand there

is the environment, which seems to be continually acting upon that individual. What is the connexion between the two ? It is obvious that the organism is dependent to a certain extent upon its surroundings. But there is still this question to be answered : do any of these influences alter the essential nature of the organism, or are all the changes brought about by the environment, merely casual and superficial modifications of an individuality which in itself does not change ? Most modern biologists seem to be of the opinion that the surroundings (i. e. Nurture) are capable of actually moulding the Nature of animals and plants. Unfortunately, however, this opinion does not represent an idea which has been arrived at by any process of independent reasoning ; it has sprung from the tendency to 'explain' the evolution of species by referring to the inheritance of acquired characters. Had not this tendency given a certain bias to the observations of scientists, who knows if they would have held so tenaciously to this realist doctrine ?

To Darwin and his followers the problem presented no difficulties, for they did not think of the organism as a spiritual entity. The plant, the animal, or the man, was for them merely an accumulation of modifications acquired by chance. Each such accumulation increases throughout the whole individual life and is passed on to the descendants. Darwin's views on variability depended on his conception of the organism. He looked upon it as a mere passive agent, exposed to the influence of a varying environment. Hence for him there was no appreciable difference between a mutilation, by which the organism quite passively suffers a change in bodily form, and an inborn variation.

Weismann, in his discussion of this problem, emphasized this difference. He thought that inherited variations are due to forces residing within the organism, while acquired ones are due to the action of the environment. Darwin's original assumption that there is no essential difference between the two is, however, still upheld. Thinkers of this school will not admit that inborn variations represent something absolute, and not traceable to external causes ; but they regard these variations too, as brought about by the influence of the environment ; in these cases the in- fluence of 'Nurture' upon the organism must be more pro- found, however, and the connexion between this 'Nurture' and the resulting change in the 'Nature' of the organism must be more complicated.

The mechanistic conceptions of nineteenth-century biologists led them to recognize quantitative variations only. It is true that they distinguished between varia- tions which affect the form (changes in size, colour, shape, bodily proportions, the number of limbs, and so on) ; those which affect the function (as when, for example, an animal becomes accustomed to new food) ; those causing mental changes (e.g. training); but they traced back all these modifications of the organism to displacements of certain substances in the egg, in the alimentary canal, or in the brain; displacements which, small in themselves, led to much greater changes in the mature organism.

These views are apparently in the process of decline to-day; in vain do we strive to bring Darwin's original assumptions, that the organism is absolutely passive in its reaction to the influences of environment, and that the resulting variation is quantitatively proportional to the cause, into harmony with the facts. Vainly we try, by various subtleties of argument, to avoid admitting that the organism is an active 'self-controlling' agent. It hardly occurs to any one now to place a mutilation, or an increase in weight which is the result of better nourish- ment, in the same category as changes like seasonal dimor- phism, although we speak, in all these cases, of the action of the environment.