THE DARWINIAN THEORY to Darwin those naturalists who advo cated the theory of transformation laid stress sometimes on the cumulative effects of external conditions, sometimes on the action of assumed internal factors, and at other times on the guiding influence of more mysterious perfecting principles. All such speculations failed to convince, being either obviously in adequate, or calling in evolutionary forces of which no scientific explanation could be given and the existence of which could not be proved. Lamarck insisted on the direct effect of environment as the main cause of evolution in plants, but to account for that of animals brought in other factors such as their direct response to new needs and new habits. He conceived that efforts, habits, use and disuse of organs induced changes in the individual which could be inherited by the progeny and so lead to' transformation and evolution. But his theories were not worked out in scien tific detail, involved many assumptions and some contradictions, and failed to carry conviction (see LAMARCKISM) .
It was Charles Darwin who, by the patient collection of a vast array of significant facts concerning all sorts of organisms, living and extinct, first clearly showed in his immortal work The Origin of Species (I 8S9) how overwhelming is the evidence that evolu tion has really taken place, and how impossible it is to account for the facts on any other hypothesis. But his great merit is to have made it clear that evolution may be accounted for as the result of "natural causes," which can be seen at work at the present time, can be tested by observation and experiment, and leave no room for any mysterious governing causes or interfering forces in addition.
Darwin and Wallace simultaneously discovered the great prin ciple of natural selection, the keystone to the Darwinian explana tion of the method of organic evolution, which may be described in Spencer's phrase : the survival of the fittest in the struggle for existence. It was shown that organisms vary, that these variations may be inherited, that—in competition with others—those which vary in an advantageous direction are most likely to survive and to leave progeny behind them, and that this must inevitably lead to a cumulation of variations and evolutionary change. According to Darwin, then, the chief factors which contribute to the process of evolution are variation, heredity and the struggle for existence. To their combined action he gave the name "natural selection" in analogy with the very similar process of artificial selection carried out by man on domestic plants and animals. For the breeder, by continually selecting and breeding from those individ uals which varied in directions favourable to his purpose and fancy, has been able to transform their characters (bodily and mental) and so bring into being various races differing from each other and from the parent stock even more than do species in nature (see SELECTION) .
While the validity of the theory of natural selection cannot be denied, its importance and that of the different factors con tributing to it in the general process of evolution have been and still are variously estimated. These factors may now be examined in more detail.
Variation from the mean is universal ; no two individuals are exactly alike in every detail however closely they may be related. Parents and offspring, brothers and sisters differ from each other, and the differences are called variations. The characters of or ganisms are all those qualities and properties whereby we can describe them and distinguish them from each other, such as shape, size, colour, mental capacities and so on, and all these are variable. Variation, in fact, affects every sort of character, struc tural or functional; and occurs at every stage of life. If these variations are measured and arranged in order of size, from the smallest to the largest, it is found that the medium measurements are the most frequent and that measurements became rarer and rarer towards the two extremes. In other words, out of a number of individuals closely related or belonging to the same species, the majority approach the mean, and the more they deviate from the mean the rarer they are (see VARIATION, BIOMETRY).
Coming now to the cause of these variations, we find that they are really of two kinds and due to different causes; but to under stand the nature of variability we must analyse it further in the light of heredity. When a character of the parent reappears in the offspring it is said to be inherited. There is, of course, a physical basis, a mechanistic aspect of heredity as of the other phenomena of life. The reason why like gives rise to like, why the reproduc tive cells of, say, a snail, a fly and a fish, all developing under the same conditions in the same pond, reproduce the same bodily structure, the same functional capacities, the same psychological powers, the same complex individuality as their respective parents, is because they are each composed of the same protoplasm as their parents. Therefore, under approximately similar conditions of environment, they are bound to develop into approximately similar organisms. This special protoplasm, peculiar to the particular organism concerned, is transmitted by the reproductive cells; there is a direct continuity of substance, and only thus can the characters of one generation be made to reappear in the next. Hence the fundamental importance of the principle of continuity insisted upon in previous paragraphs.
We may distinguish, then, two sets of factors contributing to the formation of every organism : on the one hand the substances (germ-plasm) actually transmitted, on the other hand the factors of the environment or conditions under which it develops. The former may be called factors of inheritance. Every organism and every part of it is the result of the interaction of the factors of in heritance, and the conditions or stimuli which influence its meta bolism and hence its differentiation, growth, behaviour. Every character is thus of the nature of a response to stimulus, and the characters of an organism are the sum of its past responses. Factors of inheritance are transmitted; characters, however, are not transmitted as such, but are inherited, formed anew in every generation. It is important to realize this distinction. Clearly, if both the conditions and the factors of inheritance remain the same there will be no variation and, consequently, no evolution. Varia tion, i.e., deviation from the parental form, must be due to some change, either in the factors, or in the conditions, or in both.
It is a matter of common observation that individual organisms are to some extent modified by altering environmental conditions, becoming changed when transferred from one environment to an other, by the application of new stimuli, by use and disuse. Plants offer obvious examples of such effects, and often come to vary markedly from each other owing to differences of light, tempera ture, moisture or the composition of the soil. The same species, even the same individual, may take on a different aspect if culti vated in the garden or grown in the open. Animals are likewise modified and caused to vary in structure or behaviour; but usually to a less extent, especially the more elaborately organized higher forms. For in these the metabolic processes are to a greater extent regulated by the internal environment, so that the effective stimuli are more constant and less easily altered. Yet they are no exceptions to the rule, and there are many examples among animals where conspicuous differences are induced by changes in the environment. This plasticity or modifiability, this power of responding in different ways to stimuli of various kinds, or of varying intensity, is a property of all living organisms and a cause of variation.
Now, Lamarck and the older writers assumed that such acquired modifications can be transmitted in some way from parent to off spring, that they tend to increase from generation to generation by a process of cumulation which leads to the transformation of organisms and to their evolution. Such an assumption is not justi fied. As Weismann first showed, there is no proof that modifica tions are transmitted, and no good reason to believe that they are cumulative.
An artificial distinction is often drawn between characters, some of which are said to be "acquired" as the result of the direct action of conditions, and others said to be "innate." But since all charac ters, as explained above, are the result of the interaction of the transmitted factors of inheritance, with the environmental stimuli encountered, this distinction cannot hold good.
It may be concluded that the reason why two organisms re semble each other is because they start with the same complex of germinal factors of inheritance and develop under the same environmental conditions. Hence, if either the factors or the conditions are altered, variation will result. What may be called the normal bodily and mental structure is that which develops under the usual complex of environmental stimuli. When some deviation occurs and this variation is due to a change in condi tions, it may be called a modification; when it is due to an altera tion in the factors of inheritance it may be called a mutation. Thus, although there is only one kind of character, there are two causes of variation. Both may give rise to new characters which may be inherited and reappear in following generations. The mutation will necessarily reappear if the environment remains unchanged, since the factors of inheritance are actually trans mitted. So will the modification reappear, although not trans mitted, if the conditions which called it forth persist, if the necessary stimuli are present; but it will not reappear if these stimuli are absent. It follows that a new mutation will be inherited in a constant environment, but that the inheritance of a new modification depends on the presence of the necessary conditions. The point of prime importance to notice is that mutations, being due to lasting alterations of the factors of inheritance, will be persistently inherited ; and by the addition of new mutations may lead to evolution.
To avoid ambiguities, many authors now use the terminology of Johanssen : the material factor of inheritance is called a gene ; the word genotype is used to signify the whole complex of genes possessed by an organism, and the word phenotype to denote the form derived from it, the sum of its characters. The phenotype is moulded by the environment ; different environments will give rise to different phenotypes. The more plastic or modifiable the genotype, the greater will be the number of possible phenotypes and the greater the difference between them. Genotypes are trans mitted, phenotypes are (may be) reproduced.
In sexual reproduction the specific substance containing the essential factors of inheritance must be transmitted in the germ cells, and it has now been shown that it is present in their chromo somes, and is carried by the gametes of both sexes, since inheri tance is equal from both parents.
If two individuals of a species differing from each other by some contrasting character, say a white-flowered or a red-flowered snapdragon, are crossed, their progeny will have pink flowers unlike those of either of the parents. If, now, two of these pink-flowered "hybrid" or generation, are interbred, they will produce off spring generation) of three kinds, with white, pink and red flowers, and in the proportion of one : two : one respectively. The white-flowered individuals will breed true and continue to breed true if interbred, and similarly the red-flowered individuals; but the pink-flowered will never breed true. If the pink are interbred they will, at every generation, give rise again to the three kinds and in the same proportion. It is concluded that the appearance of the white or the red character in the phenotype is due to the presence in the zygote of either white-determining or red-deter mining factors transmitted from the parents ; and that the appear ance of the pink character is due to the presence of both a white and red-determining factor in the same zygote. Further, it is con cluded that the factors in a zygote are in pairs, one received from the paternal and one from the maternal parent, that in the forma tion of gametes only one kind of each of the pair of contrasting or alternative factors can pass into each gamete, and that, there fore, the factors are segregated and so distributed to the individual gametes. The zygote or individual bearing the pair of similar factors is called a homozygote, that bearing the pair of dissimilar factors is called a heterozygote. That the above conclusion is correct may be verified by "back-crossing" a heterozygote with a homozygote, when the resulting offspring will consist of homo zygotes and heterozygotes in equal numbers. Frequently the heterozygote resembles one or other of the parents more or less completely; for instance, all the offspring of a cross between a black and a white (albino) mouse will be black. In such cases that factor of the pair which is expressed in development so as to mask the effect of the other, is said to be dominant, and that factor whose effect is masked or prevented is said to be recessive. A mouse homozygotic with regard to factors determining black ness is a pure dominant, one homozygotic with regard to factors producing whiteness is a pure recessive.
It follows that an organism can only exhibit those characters for the development of which it has the necessary genes. The presence of dominant genes may be inferred from its characters ; however, since some genes may be recessive, the total factorial equipment of an organism cannot be told from its characters alone, but only by the results of breeding which will bring out the recessive char acters in the "recessive" homozygote individuals.
It follows also that any mutational alteration of factors will necessarily be transmitted and perpetuated unaltered, unless some fresh mutation supervenes ; and that the factors are not altered, nor their effects necessarily swamped or diluted by intercrossing, however prolonged. Two important conclusions affecting the gen eral theory of organic evolution follow. In the first place it is the character, not the genes, that are directly selected in natural as in artificial selection. The suitable phenotype is successful in the struggle for existence, and the genes are only indirectly selected in so far as they correspond to it. Secondly, the modifiability of an organism is necessarily strictly limited by the potentialities of its complex of genes. It may become modified in various directions, since various environmental conditions may call forth a response, and this may vary in amount according to the strength and dura tion of the stimulus. Once the maximum response has been reached, however, it cannot go further. Since modifications are not cumulative, it is the mutational variations, due to some altera tion in the complex of genes, that are important in enabling evolu tionary transformation to take place. By the cumulation of fac torial changes the lasting and presumably unlimited alteration of characters can be brought about.
How great are the possibilities of increase, and how effective are the natural checks to reproduction, may be seen when the bal ance is disturbed and some of the checks temporarily removed, as, for instance, owing to a change of climate or the intervention of man. As examples may be mentioned the swarms of lemmings, and of locusts which, occasionally or periodically, sweep over vast regions; also the great epidemics of diseases. Similar expansion of one species at the expense of others is continually going on on a small scale, even in the most stable flora and fauna. The extraor dinarily rapid increase of a species, introduced into a new coun try where it does not meet with the ordinary checks occurring in its native habitat, is seen in the case of the European sparrow in North America, the rabbit in Australia, the Phylloxera insect which almost exterminated the vines of Europe. The spread of water-cress blocking the rivers of New Zealand and of the Amer ican water-weed those of Europe, the devastating conquest of vast areas in Australia by the prickly pear, are examples from the plant world.
The intensity of the struggle varies in different seasons, some times periodically. In a stable population on the average only two individuals can survive to replace the parents out of the whole number of offspring. As a rule the greatest destruction takes place when the organisms are quite young.
This constant struggle for existence is the primary factor in the process of natural selection whereby adaptation is brought about. The resulting selection is between the variations which are in all possible directions, as already explained ; some will be ad vantageous, some disadvantageous, and others perhaps neutral. The first test every organism has to pass is that of viability, the power to live at all. Just as the intensity of the struggle may be measured by the death-rate, so a variation is advantageous in so far as it lowers this rate ; and this gives a measure of its "selection value." Selection acts like a sieve, separating individuals best en dowed to survive from others less fortunate. It may be repre sented equally well as the survival of the fit, or as the elimination of the unfit. In the long run those organisms which vary in the "right" direction will succeed and leave offspring behind them, while the others will be crushed out. In so far as the advantageous variations are due to lasting factorial changes capable of being transmitted and accumulated by the piling up of fresh changes, evolution will continue in the direction of adaptation. Only thus can the adaptation of living organisms be intelligibly explained, and this is the great merit of Darwin's theory of evolution by natural selection which has secured its triumph over rival theories.
It should be clearly understood that the theory does not explain variation, but accepts its existence as affording the material for selection to work on. As Darwin insisted, without variation nat ural selection can do nothing. What selection alone can do is to pile up individual favourable differences so as to bring about ever increasing divergencies along various lines of adaptation. And so, step by step, are produced those marvelous adaptations, those wonderfully perfected and complicated organs familiar to all students of plants and animals. Moreover, this affects not only the structure of the parts of individuals, but the relations between individuals which may become mutually adapted to their advan tage, as in the case of parent and offspring, and of animal "soci eties." Further, it may lead to the development of mutual adapta tion between different species to the benefit of both, of which striking examples occur among both plants and animals. One may instance the "symbiotic" association of colourless Fungi and green Algae, known as lichens (q.v.), or of green flagellates living in the tissues of the common Hydra. Perhaps more wonderful still are the mutual adaptations of insects and the flowers they pollinate, a subject to the elucidation of which Darwin contributed so much. Indeed, the intricate interrelationships of the various organisms inhabiting a given area, their influence on each other's abundance and distribution is extraordinarily complex, and forms the chief subject of a special science, ecology (q.v.).
If elimination is to be effective in evolution, if it is to bring about adaptation, the death-rate must be selective; i.e., apart from accidental extermination, the successful survivors should in the long run differ from those which fail. It is easy enough to demonstrate this in the case of larger groups differing by conspic uous characters, and even sometimes of "species" or "races" of which we know the history; but it is of ten very difficult to prove effects of natural selection at work among closely related individ uals differing only by small or, it may be, by invisible characters. Rarely can the failures be directly compared with the successes. Nevertheless some attempts have been made to obtain this direct evidence : for instance, Weldon has shown that in the terrestrial mollusc Clausilia, extreme variants are eliminated, while those approaching nearer to the mean of the species are preserved ; and Tower obtained the same result with the beetle, Leptinotarsa. Harrison has given good evidence that, following on the separation of a wood of pines and birches into one half consisting almost entirely of dark pines and the other of silvery birches, of two marked varieties of the moth Oporabia, the light variety has been almost eliminated from the pine wood and the dark variety from the birch wood by the bats and birds which feed on them. Of indirect evidence there is overwhelming abundance. One may here again mention the close adaptation of most forms to their environment ; the wide-spread occurrence of protective resem blance between organisms and their surroundings whereby many escape the notice of their enemies; or of warning coloration by means of which species advertize their unpalatability ; the failure by constant elimination of certain common extreme variants, such as albinos, to establish themselves ; the development of immunity to those particular fatal diseases with which a species comes in contact. No one who has studied living organisms in their natural surroundings, especially in the tropics, can doubt the severity of the struggle, the reality of selection. Here may also be mentioned those striking resemblances between species, especially of insects, sometimes closely allied, but often of separate orders, due to mim icry (q.v.). No other explanation of these often astounding sim ilarities in appearance of structure and coloration seems possible but that of gradual convergence through the selection of suitable variations.
What, it may be asked, is the effect of selection on succeeding generations, what part is played by elimination and inheritance in evolution? This question has already been partly answered above when it was pointed out that to be effective, selection must be between individuals differing in their factorial endowments. Among individuals homozygotic for the factors determining a given character, selection of variations of that character due to environmental conditions can have no cumulative effect. But natural species are rarely, if ever, so uniform in their factorial inheritance; on the contrary, they differ, as a rule, as regards a considerable number of factors. Selection of individuals in which a given character is best developed, will then rapidly sort out a strain possessing the most favourable assemblage of factors. But once this strain has been isolated, further selection can do nothing unless some new mutation in the required direction arises.
No arbitrary limit can be set to the "selection value" of a character; its value depends on the intensity of the struggle at the moment. The slightest difference in weight between two seeds carried by the wind may decide that one will reach a favourable spot and not the other; the smallest difference between the staying powers of two animals in a flood may enable one to swim to safety and not the other; the smallest inferiority in powers of resistance to disease may cause one man to perish while another recovers. Moreover, a character, useless during the greater part of the life of an organism, may prove of vital importance at a particular time.