Chapter XXIII: Appendix: II 488 (3)

The facts regarding the conditions of life and the dangers to which the young of different species are exposed coincide with the view that the strength of reproduction is in the main determined by the sum of these dangers. The greater strength of fecundity among those species which shed their eggs into the water cannot be attributed, as we have seen, to any failure of fertilization on a large scale; it may be attributed, however, to the numerous dangers which the young of such species must encounter. The chance that any single fertilized egg will grow into an adult is far less in these cases than when the eggs are retained in the body of the mother and when the young are guarded by the parent, and, unless fecundity was on a large scale, a sufficient number would not survive. Among plants reproduction must always be on a large scale because it is necessary to ensure that a sufficient number of seeds will fall, not only on suitable ground, but on suitable ground that is not so occupied as to prevent growth to the adult stage, in addition to the necessity of providing such numbers as will ensure a sufficient number passing through all the other dangers. Eggs and young abandoned by the parents may be variously exposed to danger. The development of instincts common among insects which lead to the hiding of eggs in places where danger of destruction at the hand of enemies is decreased, and to the provision of food for the young when they develop from the eggs, is accompanied by a decrease in fecundity. Further developments of the parental instinct are accompanied by decreases in fecundity as the degree of danger lessens. Parental care for the young is not uncommon among the invertebrates. There are a number of cases in which the young are retained in brood-pouches, as for instance by the common water-flea. The fresh-water leech, Clepsine, broods over its young. Among the vertebrates the fish for the most part take no care of their young. Some fish, however, such as Gasterosteus, the Stickleback, make nests. No great advance is found among the Amphibia or among the reptiles. One reptilian group, however, the Chelonia, shows a remarkable advance; they live in pairs and guard their young with care. If marriage be defined as ‘a more or less durable connexion between male and female lasting beyond the mere act of propagation till after the birth of the offspring’,[96] then the beginning of marriage is discernable in this group. This more or less durable connexion between male and female is a well-known characteristic of birds, as is also care for the young. ‘Most birds when they pair do so for good and all until one or the other dies.’[97] The connexion is not so durable, and the parental instinct is not so highly developed, among mammals as among birds. The retention of the young within the body of the mammalian mother, however, greatly decreases the dangers to which the young are exposed. It is also worthy of note that those species both of birds and mammals which prey upon others are on the whole less fecund than other species, a fact which is to be connected with the lower degree of danger to which their offspring are exposed.

The lower degree of danger to which the young are exposed, the less the fecundity; and the less the fecundity, provided that it reaches the strength necessary to preserve the species, the better on the whole for the species. In this manner we reach the second generalization regarding the quantitative aspect of the population problem among species in a state of nature, which may be stated by saying that the strength of fecundity in any species is determined by the sum of all the dangers to which the young of that species are exposed. This must be qualified, in so far as fecundity and fertility are not the same, by adding to the dangers to which the young are exposed the danger that a certain proportion of eggs will not be fertilized. It follows that among men, since fecundity and fertility are not the same for quite other than ‘mechanical’ reasons, fecundity is not directly related to the dangers to which the young are exposed. It is clear that, when men, as is now the case on a large scale, both abstain from intercourse and interfere with the natural result of intercourse, and at the same time increase in number, the strength of fecundity is considerably greater than that which would enable a sufficient number of young to escape the unavoidable dangers. But the ancestors of man must once have been subject to the same conditions as those to which species in a state of nature are now subject, and it will be into the causes and results of this progressive divergence from the former conditions that we shall look in the first part of this book.

III
THE BASIS OF THE POPULATION PROBLEM: (2) THE QUALITATIVE ASPECT

1. We have now to consider the qualitative aspect of the population problem. There are two questions to which an answer is required. First we ask: What is the nature of the changes which occur? In other words, if we may speak of history among animals and plants, we want to know what kind of changes underlie the facts which go to make up that history. The second question is: How have these changes come about? There is no doubt about the answer to the first question, and it can be given very shortly. Though there is a certain consensus of agreement regarding the answer to the second question, there are considerable differences, of opinion over points which are not unimportant, and in order to deal adequately with these points a long discussion would be required for which there is no space. All that can be done here is to set out the most important facts; they will provide a satisfactory answer to the first question and enable some indication to be given of the lines which the answer to the second must follow.

However slight the treatment of the problem, it must begin with a reference to the physical basis of inheritance. Some description has been given of the process of fertilization. It was said that only the head and middle-piece of the spermatozoon penetrate the egg, and that no cytoplasm or ordinary granular protoplasm can be demonstrated in these parts of the spermatozoon. The head is the nucleus of the cell, while the middle-piece contains the centrosome, a body which is attached to the nucleus and plays an important part when the nucleus divides. From these facts a very important conclusion follows. It is known that both parents contribute equally to the offspring; therefore as the male parent contributes only a nucleus and its appanage the centrosome, the basis of the inherited qualities must be sought in the nucleus.

Attention is thus directed to the nucleus, which can sometimes be seen in the living cell. Its detailed structure can, however, only be made out in specimens which have been preserved and stained. The nucleus is then seen to consist of a thin wall within which is contained a colourless sap. In the sap are a number of beads of a darkly staining substance known as chromatin suspended on delicate threads of a substance known as linin. The division of a cell is always preceded by the division of the nucleus and, when the nucleus divides, the chromatin undergoes certain remarkable changes. The beads of chromatin become aggregated together into rods—the number of rods which appear being invariably constant in the same species, though varying from species to species. These rods are known as chromosomes. Beyond saying that the rods divide into two and that each daughter nucleus, and therefore each daughter cell, is provided with that number of chromosomes which is typical for the species, it is not necessary to follow the details of the process of division any farther. Stated in the briefest possible form, this is what happens during ordinary cell-division, such as that which takes place when a fertilized egg is growing from a single cell into a multicellular adult.

There is one remarkable exception to this type of nuclear division. In the last division but one of those series of divisions which lead to the formation of both male and female gametes, half, and not the full number of chromosomes, is transmitted to each daughter nucleus. If therefore the typical number of chromosomes in the nucleus of one species is eight and in another four, the number of chromosomes in the gametes will be four and two respectively. In the former species the nucleus of the egg will have four chromosomes, and that of the spermatozoon also four, and the full number typical of the species will only be restored when the nucleus of the spermatozoon fuses with the nucleus of the egg in fertilization.

The invariable reappearance of the same number of chromosomes in ordinary cell-division, their reduction to half that number in the last division but one which precedes the formation of the gametes, the complicated mechanism which is employed and other evidence all lead to the conclusion that the location of the basis of the hereditary qualities can be further narrowed and sought in the chromatin—in one element, that is to say, of the nucleus.[98]

2. The nucleus must not be thought of as isolated from the rest of the cell; there is a constant and active interchange between the nucleus and the cytoplasm. It has actually been demonstrated that at times particles stream out from the nucleus into the cytoplasm. The nucleus is the centre of activity; without the nucleus the cell cannot live. This being so, how are we to view the process of development from the fertilized egg to the adult? The process can be studied in detail. The egg divides into two cells, then into four, eight, sixteen, thirty-two cells, and so on. The gradual differentiation of the organs can be watched and their lineage to certain cells in earlier stages traced. At every stage the developing organism comes under the influence of certain stimuli provided by the surrounding conditions. Very many elements in the environment act as stimuli; among them are light, temperature, gravitation, food, and so on. Provided that the environment is normal, provided, that is to say, that the variations in the stimuli do not exceed certain limits, an adult member of the species will be formed. What is implied in the term ‘normal environment’ will be more fully discussed in Chapter XIV, where the results of subjection to an abnormal environment will also be studied. That a large number of stimuli varying within only narrow limits are necessary, will there also be shown to have been proved.

The process of development, therefore, takes the shape of the growth of a particular organic form through the play of certain stimuli upon the germinal constitution. The germinal constitution has, as we have seen, apparently to be identified with the constitution of the chromosomes of the fertilized egg, of which half are derived from the father and half from the mother. It is known that the characters, which the organism exhibits, in some manner have a basis in the germinal constitution. The number of these separate characters is very large. What view then are we to take of the nature of the germinal constitution? In the first place every character as seen in the organism is the result of the play of certain stimuli upon something in the germinal constitution. This something can only be thought of as predispositions to the development of certain characters under certain stimuli and to the development of characters differing in degree or in kind from the former characters under other stimuli. It is altogether misleading to speak of anything but predispositions as present in the germinal constitution, though, of course, for the sake of brevity other phraseology may be employed once this point is understood. When, therefore, we speak of the inheritance of any character, we mean that there is a predisposition in the germinal constitutions both of the parent and of the offspring to develop this character under certain stimuli, which stimuli must play both upon the parent and upon the offspring if the character is to be manifested in both of them.

If we have to regard the germinal constitution as somehow containing very many separate predispositions or, as they are often called, factors, is it in the second place possible to say in what these factors consist? No definite answer to this question can as yet be given. Certain hypotheses have been put forward. Weismann, for instance, suggested that the factors were to be sought in groups of molecules of chromatin. It is not necessary, however, to postulate definite and separate particles as the physical basis of the predispositions. The protoplasmic molecule is a very complex structure consisting of a very large number of atoms. Similar atoms may be differently grouped within a molecule, and different predispositions may well be functions of different groupings.

3. In order that we may answer the first of the two problems set out at the beginning of the chapter we must consider in rather more detail in what the development of an organism consists.[99] Every character is, as we have seen, the result of the influence of the environment upon what is innately given. If two individuals were endowed with precisely similar germinal constitutions, and if precisely similar stimuli played upon each of them, then the adult forms would be similar in respect of all their characters. But if the stimuli are not similar, if, for instance, more food is provided in one case than in another, then, though the germinal constitutions are similar, one adult may be larger than the other. Again, let us suppose that the germinal constitutions differ, that, for instance, there is in one case a predisposition to the development of greater size than in the other, then, even if the stimuli are similar, the adult forms will differ. The larger members of each of these pairs may thus resemble one another very closely in outward characters; but this close resemblance will not be due to similar germinal constitutions. It follows that by mere outward inspection no conclusion can be reached as to the germinal constitution. There are, therefore, two classes of influences at work, and alterations in either class of influence will bring about alterations in the resulting organism; similar characters may be the product of one kind of predisposition and one kind of stimulus, or of a different predisposition and a different stimulus. It also follows that we cannot speak of certain characters as inherited and of others as acquired. Let us suppose that some departure from the normal structure occurs. It may be due to a change in the environment, that is to say, to a new stimulus acting upon an unchanged germinal constitution, or it may be due to a change in the germinal constitution when no change in the environment takes place. It should not be said that in the former case the new departure is acquired and in the latter case that it is inherited. What has happened is that in the former case a new stimulus acting upon the old factors has brought forth a new character, and in the latter case that the old stimulus has brought forth a new character because it has acted upon a different factor.

Though the popular distinction between characters which are acquired and characters which are inherited is misleading, there are, nevertheless, two kinds of variation. A new departure may be due to a change in the germinal constitution. In this case the new character will reappear in future generations, provided that the changed germinal constitution remains and provided that the complex of stimuli which composed the environment does not change. Such a variation may be called a ‘mutation’. A new departure may also be due to a new stimulus acting upon an unchanged germinal constitution. In this case the new character will only reappear in future generations provided that the new stimulus remains. Such a variation may be called a ‘modification’. Mutations, therefore, are transmitted in the germinal constitution, while modifications are not so transmitted.

4. Already we have reached the answer to the first problem. Permanent change in organic form is due to germinal change. But before we go on and ask how germinal changes, that have arisen, become established, we must consider further the difficult problem of the manner in which germinal changes arise. A reference first to what are known as ‘pure line’ investigations and afterwards to Mendelian phenomena will illustrate what is known as to the nature of existing germinal differences; for until we have some such information, we cannot profitably ask how existing germinal differences arise. The best known ‘pure line’ experiments are those carried out by Johannsen with beans. The flowers of beans fertilize themselves; the offspring, therefore, have approximately the same germinal constitution. The offspring constitute a ‘pure line’, for by a ‘pure line’ is meant a group of children which are the offspring of a single parent. The character selected for investigation was weight and it was found that, if beans were collected from a bean-field and weighed, every gradation occurred between a minimum of about 20 centigrams and a maximum of about 90 centigrams. When the beans were separated into three classes—heavy, medium, and light—sown and plants raised from them, the average weight of the beans produced by the plants derived from the heavier seed was greater, though not proportionately greater, than the mean weight of all the beans, and that the average weight produced by the plants derived from the lighter seed was less, though not proportionately less, than the mean weight of all the beans. There was, in fact, a certain regression on the part of the heavy and light classes to the mean weight. A similar tendency to regression to the mean can be observed when other characters are similarly studied. The average stature, for example, of the offspring of tall parents is slightly nearer the mean stature of the race than the average stature of the parents.

This tendency to regression to the mean has long been known, but it was not until Johannsen proceeded farther and investigated inheritance within a ‘pure line’ that it was understood. In the experiment described, no attention was paid to the ‘pure line’. When Johannsen separated the beans produced by the self-fertilization of a single plant, divided them into heavier, medium, and lighter classes, sowed them and weighed their progeny, he discovered a very interesting fact. The average weight of the progeny of a heavy bean and of a light bean belonging to the same ‘pure line’ was the same. The conclusion to be drawn from this result is that the differences in weight between the offspring of a single self-fertilized plant are due to differences in the stimuli which play upon them. Obviously the stimuli do differ; the weight is affected by differences in light, shade, number, and position of the beans in a pod and so on. But since the offspring of a single plant have approximately the same germinal constitution, the average weight of the offspring of light beans, which are light because of subjection to less favourable stimuli than the average, will not be on that account lighter than the mean weight of the strain, and similarly _mutatis mutandis_ with regard to the offspring of the heavy beans.

It follows, therefore, with regard to this particular character that there are a number of different strains in the population. The beans arising from any one strain are not of the same weight because they have been subjected to different stimuli. If each strain was pure and if differences in environmental stimuli could be removed, then beans gathered from a bean-field would not exhibit a simple gradation in weight: there would be a number of steps and as many steps as there were strains. In actual fact, the modifications produce the gradation which is observed. The fact that the nature of the germinal constitution cannot, as stated above, be determined by a mere inspection of the characters is exemplified by this experiment. If we take a bean weighing, say, 55 centigrams, it may belong to a strain which varies, say, between 20 and 65 centigrams, or to a strain which varies between 40 and 90 centigrams. It is only when this bean is sown, and the weight of the offspring calculated, that the strain to which it belongs can be ascertained. The experiment further shows why regression towards the mean is observed, when, instead of limiting investigation to a ‘pure line’, beans differing in a certain respect from the average, by, for instance, greater weight, are selected and sown. Such beans will belong to several different strains; they will, however, include more which have been favourably than unfavourably influenced by the surroundings. The average of the weight of all their offspring will therefore be less than the average weight of the parents. At this point we touch upon the problem of selection, but before we go on to deal with this question, it is necessary to say something more about variation and the origin of variations.

5. Self-fertilization is very exceptional, and what happens in the case of the beans, though illuminating, is not typical. As a general rule in reproduction, two parents contribute to the germinal constitution of the offspring. Strains, therefore, do not remain pure, as in the case of the bean, because they are continually crossed; we require to know what happens when crossing takes place. This study, first successfully undertaken by Mendel, has been greatly extended in late years. We may first illustrate in the simplest form what it was that Mendel discovered and then go on to inquire what deductions bearing upon the question we have to answer are to be drawn from it.

A large number of experiments of the following kind has been made. Two strains in any species are chosen; these strains exhibit opposed characters. Such characters may be tallness and dwarfness, colour of the flower, shape of the comb in fowls, condition of the seed, whether smooth or wrinkled, and so on. We may call one character A and the other _a_. The two strains are crossed, and in the first generation the offspring are all alike and exhibit a character, A′. This character may be the same as either A or _a_, a blend between them or something wholly new. Whatever form it may take, it is produced by the interaction of A and _a_. The members of this first generation are then interbred, and of the second generation one quarter exhibit the character A, one quarter the character _a_, and the remaining half the character A′. If the quarter exhibiting the character A are interbred, all the offspring exhibit the character A, and the same holds good regarding the quarter exhibiting the character _a_; but if the half exhibiting A′ are interbred, the offspring will split up in the same proportion as in the previous generation, one quarter exhibiting A, one quarter _a_, and one-half A′. This result holds good for any number of generations so long as interbreeding is continued.

Into the very numerous complications which occur it is not necessary to go. They are all interpreted by extensions of the simple explanation which is applied in the elementary case given above. It is supposed that all characters which behave on crossing as above are represented in the germinal constitution by factors which behave as separate units; such characters are called ‘unit-characters’, and such factors ‘unit-factors’. It is further supposed that each gamete bears one unit-factor only in respect of each unit-character. If the strain is pure, as in the case of the two strains exhibiting characters A and _a_, then all the gametes will bear the unit-factors for A and _a_ respectively, and the fertilized eggs resulting from the crosses between them will therefore contain both unit-factors. The hybrids, which, as we have seen, exhibit the character A′, will produce gametes, half of which bear the unit-factor for A and half the unit-factor for _a_. When the hybrids interbreed, on the average of chances one quarter of the fertilized eggs will have two factors for A, one quarter two factors for _a_, and one-half both a factor for A′ and a factor for _a_. In this manner the splitting up of the second generation is explained.

What is important in this explanation is the conception of unit-factors. The extension of the explanation to cover the more complicated cases does not involve any modification of principle. It follows that the germinal constitution contains a very large number of unit-factors; what therefore is innately given in the germinal constitution is a collection of such unit-factors. Each unit-character based upon a unit-factor can theoretically be separately distinguished and isolated. The complications to which references have been made are in part due to the difficulty of distinguishing unit-characters. What is apparently a simple character may not be a unit-character, but a combination of unit-characters. This apparently simple character cannot appear unless all the unit-factors, upon which these unit-characters are based, are present in the germinal constitution.

It may next be asked whether all unit-factors behave in this manner when crossed. To this no definite answer can yet be given. The successful analysis of apparently contradictory cases and the continual discovery of characters which do behave in this fashion seem to point to an affirmative answer. The suggestion is that, when this mode of behaviour cannot be demonstrated, it is because the unit-characters have not yet been distinguished and isolated.

This brief reference to ‘pure line’ investigations and to the Mendelian analysis of crossing leads therefore to the following conclusions. When, as in the case of the self-fertilizing bean, the strains are kept pure, existing germinal differences in respect of any character are found to be of the nature of steps which are usually small—though in outward manifestation the differences are smoothed over by the influence of environmental stimuli. This is the nature of the germinal differences, and they remain what they are—apart from the origin of new factors or of the loss of old factors, and apart from the effects of a differential death-rate. When, as is usually the case, biparental reproduction takes place, the position is more complicated: strains are being continually crossed, and, as the result of the chance mixture of factors in the germinal constitution of the children, the offspring of the same parents differ in their germinal constitution one from the other. Therefore, in any species in which biparental reproduction takes place, new combinations are constantly arising; but, though in this manner the germinal constitution may in a sense change, the change is due simply to a shuffling of factors. What we require now to ask is what is known as regards the manner in which new factors arise and are added to the germinal constitution, and the manner in which old factors drop out and are lost from the germinal constitution; for it is only owing to such additions and to such losses that true germinal change occurs, and that shuffling is rendered possible.

6. The interpretation of the results of the crossing of different strains has shown what kind of changes underlie the appearance of certain mutations. It has been shown that many of the varieties of domesticated species have originated by the apparent loss of one or more unit-factors.[100] Thus the numerous varieties of domestic rabbits and of sweet-peas are all descended from a single wild species of rabbit and of pea, and differ from the wild stock not by an addition to, but by an apparent subtraction from, the total number of factors in the germinal composition of the wild stock. The reasoning which has led to this conclusion need not be followed here; one proof is that when certain varieties are crossed, characters of the original stock reappear, due to the fact that, one variety having apparently lost one factor and the other another, crossing results in the re-combination of the factors necessary to the manifestation of the original character.

This is a strange conclusion, but it seems nevertheless to be true that in this manner many domestic varieties have arisen. If this was the only manner in which mutations could come about, then we should be driven to imagine that the most elementary form of life contained within it innumerable factors, and that evolution has merely consisted in the apparent dropping out of factors. This conclusion has indeed been tentatively suggested. But until it has been definitely shown that this is the only manner in which mutations originate, it must be supposed that changes in and additions to the complex of factors can and do occur. And we may note that certain distinguishing characters of domestic varieties of fowls and of pigeons appear to have arisen by the addition of factors; thus the ‘single comb’ of fowls is the original character of the wild stock which has been modified by the apparent addition of other factors and not by the dropping out of one or more existing factors. It is again not necessary to go into the reasons which have led to this conclusion. The conclusion is a deduction from the analysis of crosses between different breeds of fowls.

7. For the most part we are quite ignorant as to the causes which have led to the apparent losses and apparent additions of factors, though a few observations seem to indicate certain circumstances under which the dropping out of factors may take place. In this connexion some reference must be made to the problem as to the inheritance of acquired characters, though it follows from what has been said above regarding terminology, that we should more correctly speak of this problem as the question whether modifications in any one direction tend to be followed by mutations in the same direction. As the result of prolonged discussion and controversy it is now generally held that nothing of the kind takes place. It is almost universally agreed, for instance, that such modifications as are induced among men at the present day do not lead to mutations in the same direction. Therefore we may for the purposes of this book, so far as man is concerned, take it that acquired characters are not inherited. But looking at the problem as a whole it cannot be regarded as settled. There is, for example, some evidence of the parallel induction of modifications and of mutations; but the question may be raised whether such cases if substantiated fall under the heading of the inheritance of acquired characters. Again, some biologists regarding the problems of evolution generally find difficulty in arriving at an explanation unless under certain circumstances adaptive variations are followed by mutations.

We are thus left with the fact that mutations arise, and may be either large or small. Outwardly, variations in characters are usually continuous, because the environmental stimuli vary continuously, smooth over and obscure the differences due to mutation, as in the case of the beans mentioned above. But, discounting the influence of the environment, the mutations themselves may form a series separated by steps that are so small as to be scarcely measurable or which may be very large. The weight of the beans is an example of the former kind of mutation; the so-called ‘meristic’ variations, when another member is added to a series, the addition, for instance, of a vertebra to the vertebral column, is an example of the latter kind.

Whenever a mutation occurs, we have to think of it as founded upon some change in the germinal constitution. Such changes are of the nature of modifications of factors—and may be positive (leading to the apparent addition of factors), negative (leading to the apparent loss of factors), or qualitative. But of the nature of these changes we know little, and of their causes less. What is important, however, is that these changes do occur. Further, they occur in all directions. The direction is, of course, in a sense determined by the starting-point—by what is already given in the germinal constitution—but, given the starting-point, mutations apparently occur in all directions. It is a matter of importance to know whether mutations ever tend to occur more in one direction than in another. Nothing definite has been ascertained as to this particular problem, though, as we shall note later, certain facts with regard to the evolution of animals suggest that there have been tendencies to change along certain lines. It may also be asked whether the continued selection of a character in any way affects the direction of the variation of the germinal constitution. To this again there is as yet no definite answer. On the whole it is not probable that selection has any such effect.

8. So far, therefore, as we have at present gone, we have found that permanent change is of the nature of germinal change. We have also discussed the nature of germinal differences as they exist between different individuals, and we have discussed the nature and causes of germinal change. With regard to variation there is much which is doubtful and obscure, but bearing in mind the essential features of what is certainly known, we may turn to consider what passes in the organic world. Upon this subject there is much less uncertainty. We have seen how huge is fecundity. Of the young in any generation, only about that number survives which equals the number of adults in the generation from which they are derived. For normally in the organic world the total number of adults of any species at corresponding periods in succeeding years remains much the same. The instances of a marked increase in the number of any species are rare, and are usually traceable to the intervention of man, as the result of which certain of the dangers which normally confront the young have been removed. It follows, therefore, that normally of the many thousands of fertilized eggs of a fish, for example, all but two perish. Even when the most rapid increase ever observed is taking place, the great majority of young perish. From a consideration of the circumstances it is evident that upon the whole those individuals which present certain characters will have a better chance of surviving than others which do not present these characters.

This point demands further consideration, because upon it turns the whole question of natural selection. What is involved in this theory is that the death-rate is selective, that those individuals which are best adapted to the surroundings which confront the species do, on the whole, have a better chance of survival. It is adaptation which determines fitness, but the concept of adaptation does not of necessity include any idea of progress. Given any complex of surroundings such as that which confronts any species, there may be a more or less close fitting of the organisms to this complex. The closeness of this fitting may have been obtained by a simplification of structure, a complication of structure, or it may be that for long ages the closeness of the fitting has been attained by the elimination of departures from the mean of the species in any direction and the preservation of the average type.

In this connexion it has often been pointed out that the death-rate in certain cases is not selective, and does not therefore involve the survival of the more fit and the elimination of the less fit. When the whale opens its mouth and engulfs vast numbers of small organisms upon which it feeds, there is apparently no escaping of certain types of these organisms accompanied by a greater elimination of other types. But these cases are not on the whole common, and further there is nothing in such cases which counteracts selection; it merely means that sometimes selection is not operative. A consideration of the mode of operation of the factors of elimination, the general nature of which has been indicated, leads without any doubt to the conclusion that in the vast majority of cases, when any organism meets its death, either that organism does not possess some character which other organisms possess and which have enabled them to survive, or it possesses some character which other organisms do not possess, and which at some crucial moment has told against it. This subject could be considered at great length, but it must suffice to say here that the more experienced naturalists are, and the greater the knowledge they have of the conditions of organic life, the more it is borne in upon them that the death-rate is upon the whole selective, and that the best adapted types have a better chance of survival than other types.

From the nature of the case it must be difficult to obtain statistical evidence of natural selection. There are, however, certain cases known which are of great interest. Bumpus, for instance, after a storm in America collected 136 sparrows, all of which had been injured. Of these 72 recovered, while the remainder died. He weighed and measured all the specimens and compared the figures for the survivors with the figures for those which had perished. It was found that the average type of the latter was larger and heavier than the average type of the former. It was also observed that there was a less wide range of variability among the survivors than among the dead, showing that the favoured type approximated more closely than the others to the average type of the species. This favouring of the average type has been shown to occur by other observations, and it may be supposed that normally it is the average type of any species which is best fitted to the particular niche in nature occupied by the species, and that therefore adaptation will be measured by the nearness of approach to this type. When, on the other hand, circumstances are changing, some deviation from the average type will be favoured.

9. This leads us to ask in more detail what it is that happens in the process of selection. Selection has been likened in its action to a sieve separating the fit from the unfit. But fitness is measured by the characters exhibited, and these characters may be either of the nature of mutations or of modifications. What is exhibited is selected quite apart from the underlying nature of the character; but it is only when variations of the nature of mutations are selected that there results any change in the composition of the germinal constitution of the species. So far as selection is merely that of modifications within a strain, there will be no change in the average germinal constitution of the progeny. It is only in so far as selection is a selection of strains that there will be any change. If the strains among the beans referred to above can be isolated, then the strain with the predisposition towards the greatest weight can be isolated, the other strains eliminated, and the average weight brought up to that found in this strain. If the strains cannot be isolated, then an approximation only to this result can be made by the continued selection of strains of the greatest weight, and this is what occurs in nature.

This leads to the most important conclusion of all. Selection can only act upon what is given. We may put aside the possible but quite unproved influence of selection upon the direction of mutation. Apart from this, selection can have no other effect than to eliminate certain strains and to favour others. Supposing that under changing circumstances a certain character is favoured, all that can happen through selection is that the strain exhibiting this character will be favoured and all other strains gradually eliminated. Further than this change cannot proceed until mutations have occurred in the direction of further increase in this character. As has been said, selection is no more than a sieve; as a rule it is occupied in separating out the extreme types and favouring the mean, but at times under certain circumstances it will favour some type that deviates from the mean, but it can do no more than favour what types are at any given moment in existence.[101]

10. We set out to answer two questions regarding quality. We asked in what change among species in a state of nature consisted. The answer to this is clear; it consists in changes in the germinal constitution. If we can speak of history in connexion with species in a state of nature, then their history is based solely upon germinal change. We also asked how change came about. It is less easy to answer in a few words. Obscure as are many of the details connected with the process, there is a sufficient measure of agreement regarding the main facts to enable an answer shortly to be formulated. The answer rests upon two series of facts. The observed characters of organisms are based upon certain predispositions in the germinal constitution. These predispositions or factors under the stimuli of the environment give rise to the various characters. These factors are derived from both parents through the fusion of the gametes, and the complex of factors in the offspring is the result of a chance mixture of factors. There may further at times be a dropping out, an addition to, and perhaps a modification of the factors. Secondly, the death-rate is selective. Although the result of selection is confused by the effect of modifications which smooth over the differences due to differences in the germinal constitution, the effect is that individuals with certain factors are favoured and others with different factors are eliminated. Therefore the average nature of the factors may be changed should any type, other than that approximating to the normal type, be favoured.

It is possible in this manner to understand how change has come about. There are many difficulties, but they are all capable of fairly satisfactory explanation. None at least is insuperable. There is one problem which is perhaps somewhat difficult to solve unless a particular supposition is introduced. This problem is connected with the evolution of organs along certain lines, as, for instance, the evolution of horns and teeth in the vertebrates. This class of evidence has chiefly impressed itself upon palaeontologists, and one of the most distinguished of them has stated that from this class of evidence he concludes that ‘there are fundamental predispositions to vary in certain directions’.[102] This is the supposition which it may be necessary to introduce, but which does not in any way conflict with what has been said.

IV
THE POPULATION PROBLEM AMONG MEN

1. It has been shown in the last two chapters in what the population problem among species in a state of nature consists. With regard to the quantitative aspect in particular it has been shown that among such species mental evolution, greatly as the stage reached differs between the lowest and the highest forms, has not attained a point at which the essential features of the position have in consequence been affected. Enough has been said to show that in the case of the most primitive races of man now living mental evolution has reached a point in consequence of which the quantitative problem has assumed a totally different aspect.

With regard to the qualitative problem the question also arises as to how far mental evolution in man has affected the position. In one very important respect mental evolution has wholly altered the position of man as regards this aspect of the problem, as it has regarding the quantitative aspect. How this has come about may be considered after it has been observed that mental evolution has also been the cause of other changes which, while they do not alter the fundamental position of man as regards the qualitative aspect, yet are of great importance.

As we have seen among species in a state of nature change is founded upon germinal change. So, too, among men there is germinal change, and so far as history (using the term in the widest sense to include what is often rather meaninglessly called ‘pre-history’) is connected with germinal change, so far it is of essentially the same nature as change among other species. But both the direction and intensity of germinal change among men have been greatly influenced by mental evolution. Certain causes of elimination have been removed wholly or in part, others have been introduced. The facts are familiar; there is no need to labour the point. It is also worth noting that, should it be found that mutation frequently arises owing to alterations in the environment during the formation of the germ-cells, such a discovery might have considerable bearing upon the position in man. Owing to human activity under the guidance of reason, the environment has been profoundly changed in many directions, and without question in consequence of such changes the germ-cells of human beings are subject during their development to far more varied stimuli than are those of other species. This, however, is only a possibility. Nothing is known with certainty upon the subject. It may be that the great variety of foods, the absorption of alcohol and nicotine, the various occupations and customs, and many other factors, all ultimately traceable to reason, may, some of them in some way, tend to bring about mutation in man.

It is by making possible the development of tradition that the evolution of reason has fundamentally changed the nature of the qualitative problem among men. Into the nature and origin of tradition it will be necessary to go in another chapter. Here all that is required is an indication of its main features in order that its connexion with the problem of change may be understood. In the stage of conceptual thought reached by man, the formation of free ideas is the outstanding feature. With the development of conceptual thought went the development of language. By language ideas can be passed from one man to another and also from one generation to another. There are other ways in which the results of reasoning can be handed on, but they need not be considered at present. What is important is that they can be and are handed on. Let us consider for a moment the question of skill. Some improvement may be made in the methods of hunting or of fishing. It may, of course, be lost, but it may be, and perhaps usually is, transmitted to other men by the inventor and afterwards to succeeding generations. Tradition is, in fact, cumulative. Even among the most primitive races now existing there is a huge mass of tradition. Succeeding generations do not necessarily start at the beginning again. They start with the experience of the race behind them, so far as it has been preserved.

Among the higher animals there is a certain handing on of what has been learnt by experience; to this extent there is tradition also among them. We shall return to this point later. So relatively unimportant, however, is tradition among other animals, that we may for the moment regard tradition as something which is found among men only. It is clear that tradition has played some part in any case in producing the changes which we call history. We have only to think of the rise of Japan to the position of a great power in the last half of the nineteenth century. The rise of Japan was an outstanding fact in the history of that period and it was clearly in the main, if not wholly, a traditional and not a germinal change. It took the form of a rapid absorption of European tradition. The evolution of reason has thus introduced into the problem of the causes of human history a factor which is not present in the case of other species; it has also modified the course of selection, but this, as we have seen, has not made a fundamental difference between the position of man and that of species in a state of nature.

The problem before us is therefore as follows. Owing to the fact of reproduction the population problem in both its aspects exists for all species in a state of nature and further presents fundamentally the same features for all such species. The ancestors of man were at one time subject to the same conditions from which they have, step by step, moved away owing to the development of the faculty of reason. We have to trace the causes and results of this moving away—of the progressive modifications of the conditions existing among species in a state of nature.

Though the problem has two aspects, they are closely interwoven. Changes which affect numbers also influence the quality of population. The discussion has hitherto taken the form of an introduction to the problem as a whole, and the two next chapters, the subject-matter of which will be indicated in what follows, will also be devoted to certain problems which equally bear upon both aspects of the question. From the seventh chapter onwards the two aspects are treated independently; we deal first with the quantitative and then with the qualitative aspect. Nevertheless we shall, when dealing with the quantitative problem, present evidence which we shall consider again later when treating of the qualitative problem. The book thus falls into three parts; the first six chapters are introductory to the problem as a whole, the next six chapters are concerned with problems of quantity (though many of the facts brought forward will be found also to bear later upon quality), and the following nine chapters with problems of quality. The last chapter sums up our conclusions as to the whole problem.

2. We may next ask what data are required in order that we may look into the changes away from the conditions under which the pre-human ancestor lived towards the conditions which now exist. In order that we may consider the quantitative aspect of the problem, it is evident that we require some knowledge regarding fecundity and as full details as possible regarding the factors which bear upon fertility and elimination. It is only when we are provided with such information that we can hope to be able to determine how it is that numbers are regulated among men.

In addition to such information, it will be desirable to have information regarding various social customs and the general conditions of life. As regards the inquiry into the qualitative part of the problem, it is clear that facts regarding fertility and elimination are again of value inasmuch as they throw light upon the nature and intensity of selection. For the second part of the problem it is also necessary to have in mind the main facts regarding the changes which are summed up as history into the causes of which we have to inquire; and in addition it will be necessary, in order that we may attempt some estimate of the relative importance of change in the germinal constitution on the one hand and of the other factor or factors of the nature indicated above on the other hand, to make some inquiry into the conditions which determine the nature of these latter factors. This will involve a reference to certain elementary facts of psychology together with some discussion of the origin, formation, preservation, and so on, of tradition.

If we first ask how far the data desired are available, we may afterwards go on to discuss how the facts are best presented. Our information is obviously very incomplete as regards the history of social habits and customs. Historical records do not take us farther back than at the most six thousand years, and the information available with respect to social customs, except for the latter part of the period, is very inadequate. If we take a broad view of history, written records have reference only to relatively modern times. There are two other sources of evidence upon which we have to rely and they are as follows.

Some indication of the bare outline of the course which history has taken can be gathered from a study of the fossil and cultural remains of man. The fossil remains are in the form of skeletons or portions of skeletons and the cultural remains in the shape of tools, weapons, and other traces of his mode of life that man has left behind him. It is interesting to observe that an attempt can be made to date these remains. They occur in certain strata, and estimates can be made of the length of time which has elapsed since these strata were deposited. Though a very large measure of doubt must surround any such estimates, nevertheless there is a certain measure of agreement concerning the facts which is of great interest. It is, therefore, possible to draw up a table showing these strata, together with what we should perhaps call guesses at the dates at which they began to be laid down. It can also be shown in the table what fossil and cultural remains are found in the respective strata.