Chapter XXII: Appendix: II 488 (2)

The manner in which these cells, known as gametes, are given off by the male and the female and the manner in which they fuse require some further explanation. What happens in plants is in all important features similar to that which happens among animals, and we may confine our attention to the latter. In every normal member of every species there is a generative organ.[74] It consists of a surrounding wall within which is a mass of developing germ-cells. In the female the fully developed gamete, called the egg or ovum, is, relative to the male gamete, of large size. It is typically a motionless cell containing a varying quantity of food substance which may, as in the case of the bird’s egg, reach a large amount. The male gamete is a much smaller cell. With a few exceptions it consists of a small oval-shaped head, to which is attached, by means of an intervening neck or middle-piece, a long vibratile tail. The head is the nucleus of the cell; if there is any cytoplasm, it is reduced to a very small amount and its presence cannot be readily demonstrated.[75] In strong contrast to the ovum the spermatozoon is typically motile and can swim in those fluids in which fertilization usually takes place.

Fertilization consists in the penetration of an ovum by a spermatozoon. The tail does not always enter the egg; as soon as the head has entered, a change takes place in the egg which sometimes prevents the tail, and usually other spermatozoa, from entering the egg. The head or nucleus of the spermatozoon approaches the nucleus of the egg and fuses with it. At this point, therefore, we have a single cell, known as the zygote, formed by the fusion of two cells, one derived from the male and one from the female. The zygote grows and divides and ultimately gives rise to an adult; into this process of growth it is not necessary to go for the moment. The zygote is to be regarded as a new member of the species the existence of which dates from the fusion of the two nuclei. Whether the zygote lives an independent existence from the beginning, or is retained within the body of the mother for a longer or shorter period, is immaterial so far as the dating of the beginning of the existence of a new individual is concerned. Every man and woman is thus in reality some nine months older than his or her nominal age.

4. Such are the essential features of the process of sexual reproduction through which new members of the species arise. We have now to consider the outward features of the process, which vary very considerably. So various are they, that at first sight it may scarcely seem likely that there is any fundamental generalization that can be made regarding the process as a whole among all species in a state of nature. When we have glanced at the facts and at their interpretation we shall be in a position to discuss what it is that holds good for all such species.

Among all multicellular animals and plants the number of spermatozoa produced is infinitely greater than the number of ova. A single normal sexual emission in man is said to contain about 226,000,000 spermatozoa. This immense production of male gametes makes it likely that a male gamete will meet and fuse with each female gamete. Among the higher animals there are certain instincts which further ensure that the male cells will be brought into the proximity of the female cells. Among other multicellular animals and plants there are no such instincts. Generally speaking, among the latter types fertilization may be thought of as fortuitous. In anemophilous plants, for example, such as the Pines, the male cells or pollen-grains are specially adapted so that they are caught and carried by the wind for long distances. Many million times more male cells than female cells are formed in these plants, and some of them, wafted by the wind, eventually light upon the ovule. So too among such lowly animals as the sea-urchin the male and female cells are extruded into the surrounding sea-water; as these animals live close together, and as the male and female cells ripen and are extruded at the same time, the chance that any egg will remain unfertilized is small, the number of the active spermatozoa being so many times greater than the number of eggs. This simple form of fertilization is characteristic of most plants and of many animals. In plants there are certain complications, in particular those connected with fertilization with the help of insects, into which it is not necessary to go.

The increasing complexity of animal structure is only roughly correlated with increasing complexity in the processes connected with fertilization. Though the most complex form of the process is found among the highest animals, yet some animals, which stand relatively high in the scale, exhibit a simple method of fertilization. Of this the fish are an example, and, further, the more highly developed or bony fish exhibit a simpler form than some of the more lowly-organized cartilaginous fish. In fact, the stage next above that of which the sea-urchin was given as an example can be illustrated from the bony fish. The process in this group consists in the approach of the male to the female alongside of whom he swims, led by a rudimentary development of the sexual instinct. When the female ejects her gametes into the water, the male does the same and the vastly greater number of the male gametes ensures that all or nearly all of the eggs are penetrated by a spermatozoon.[76]

The next stage is that characterized by external copulation. The male clasps the female and when the latter ejects her eggs, he extrudes his spermatozoa at the same time. The meeting of the gametes thus still takes place outside the body of the female. This form of fertilization is found among the Amphibia; the male frog, for example, has specially developed pads on his front feet with which he embraces the female. Internal copulation, though it is to be regarded as the most complex form of the process, is found among such low forms as flat worms,[77] of which the common tapeworm is a member, among many higher Invertebrates such as snails and insects, as well as among cartilaginous fish, birds, and mammals. The essential feature of internal copulation is that the male is provided with a special copulatory organ or penis which he inserts into the female. The male gametes pass through the penis, which takes the form of a tube, directly into the body of the female, and fertilization thus takes place within the body of the female.

5. When fertilization takes place without copulation it might be thought that a small proportion of eggs only would be fertilized, that, so long as fertilization, for instance, depends on the wafting by the wind of a pollen-grain over a considerable distance until it alights on one small spot, or upon the chance meeting of a spermatozoon and an egg in the water, there would be but a small proportion of eggs fertilized. This, however, is not so; observation shows that in a state of nature failure is comparatively rare, and that the majority of female gametes are fertilized. This is in the main due to the vast number of male cells compared with the number of eggs. Among those forms in which copulation takes place, fertilization is clearly dependent upon the strength of the sexual instinct, which impels the male to seek the female. That this instinct is very powerful is well known. It may for the time overcome all other instincts. There is a frenzy of desire among many animals. Male frogs and toads will remain clasping the female for many days waiting for the extrusion of the eggs. Female toads have been discovered smothered by the male in the sexual embrace.[78] It has been noticed that birds, which under usual circumstances are frightened by the sound of a gun, will take no notice when in pursuit of a female.[79] All the evidence points to the fact that, owing to the strength of the sexual instincts, females are rarely left undiscovered by a male in the sexual season. Jenner records that one of a pair of magpies was killed and that on the following day the survivor appeared with another mate. One of this pair was killed and on the next day the survivor again appeared with another mate. This was repeated for seven days and on each occasion the survivor always appeared with a new mate.[80] In this connexion the great development of those forms of secondary sexual characters may be referred to which enable the two sexes to find and recognize one another. Such are recognition marks, call-notes of many insects, birds, and mammals, and the strong odours given off by many animals during the sexual period.

Generally speaking, the male is always prepared for the act of copulation and the act takes place when the female is ready to receive the male. This is so among mammals whether the male experiences a sexual season known as the ‘rut’, as among stags, or whether he does not, as among dogs; for the period of ‘rut’ lasts longer than the period during which the female is ready to receive the male. The facts, however, regarding the sexual season in mammals are somewhat complicated owing to the nature of the sexual season experienced by the female. A number of different types of mammalian sexual season have been distinguished; it is not necessary, however, to go into these distinctions. It is sufficient to say that each sexual season consists of one or more sexual cycles, known as oestrous cycles. An oestrous cycle may be divided into four periods; the first, known as the pro-oestrous, is the period of preparation which ends in the rupture of the blood-vessels in the mucous membrane of the uterus. When the blood passes to the exterior, it is known as the menstrual flow. The second period or oestrous is the period of desire. This period is always short; in the sheep it lasts about twelve hours and often in other species does not last as long. Only during this latter period will the female receive the male;[81] copulation never takes place at any other period, and yet in spite of the fact that the opportunity for copulation is narrowly restricted, so strong is the sexual instinct in the male, that it is very rare for a female, so far as observation goes, not to engage in copulation at each oestrous. The period of oestrous is followed by the period of metoestrous during which the activity of generation subsides, and the metoestrous is followed by the anoestrous or period of rest, after which another pro-oestrous period begins a new cycle.

The period of oestrous, during which copulation takes place, is usually marked by the presence of ripe female gametes which are therefore at once fertilized. Ripe ova are, however, not always present at oestrous; in the rabbit ovulation takes place an hour and a half after copulation. In the bat there is a very marked want of coincidence between oestrous and ovulation; copulation takes place in the autumn, but ovulation does not take place until the following spring. In such cases the spermatozoa remain alive in the uterus until the female gametes are ripe, when fertilization is achieved; thus in the bat the spermatozoa remain alive for several months. When such cases occur, therefore, it does not mean that the ova are not fertilized. In the monkeys, however, ovulation may occur independently of oestrous and in such a manner that the ova then liberated may not be fertilized.

What has so far been said all goes to show that the majority of ripe ova are always fertilized among species in a state of nature. There must be a certain number of failures among those species which copulate, as when a mammalian female, as must now and again happen, does not engage in copulation during oestrous. Among species which do not copulate, failures are doubtless more frequent. There is probably a greater wastage of eggs when they are adhesive and fixed to some object than when they are pelagic, as is most often the case among marine animals; for when eggs are adhesive the seminal fluid may drift away.

We are now approaching a point at which it is possible to establish the first generalization regarding the process of reproduction among species in a state of nature. It is, however, not possible to make clear what it is intended to convey until something has been said regarding animal behaviour. Some of the most striking differences between one class of animal and another as regards the process of reproduction are connected with the evolution of animal behaviour. So far we have spoken of instinct without defining what is meant, and until something has been said regarding the course and limit of mental evolution among animals, the nature of the common element in all forms of reproduction among all species in a state of nature and the manner in which it differs from what is found among men cannot be set out. It may seem that the following review is taking us somewhat out of our path. It so happens, however, that in view of the questions that will come up later for discussion, such a sketch will be necessary and it may therefore be now undertaken so as to render further amplification unnecessary.

6. Putting aside the behaviour of plants, we find among the lower animals a type of behaviour which, though simple in a certain sense, cannot be adequately explained without a prolonged discussion. Some idea can be obtained of its nature if we consider in what a reflex action consists. When a definite and simple stimulus is followed directly by a definite and simple reaction we have what is called a reflex action. This term, it may be noticed, is usually restricted to cases where a nervous system is present. When similar reactions occur among organisms in which the nervous system has not been differentiated, it has been suggested that the term ‘autotype’ should be used.[82]

Reactions, however, probably never follow invariably upon the repetition of the same stimulus. Among the lowest class of animals, the Protozoa, a free-swimming ciliate Infusorian, will, if it comes into contact with something in the medium which is abnormal, it may be, for example, an alkaline solution, stop and go backwards. This is an example of a reflex action. An analysis of its subsequent movements shows that, if it again and again meets the alkaline solution, some modification of the reaction takes place. The animal may show increased activity until either it is overcome by the disturbing medium or it has freed itself from it. So again after the digestion of food, such an organism shows a different response to the presence of food.

More definite instances of different reactions following upon the same stimulus are provided by experiments of changing the nature of the medium. An Infusorian will react violently against certain new elements in the medium to which after a time it becomes habituated, when the reaction no longer follows. Very interesting experiments have shown that even among the simplest class of organisms there is a certain learning from experience. ‘A Stentor (one of the ciliate Infusorians) if gently touched upon one side will contract upon its stalk, but will soon open out again. Touched once more, it will perhaps bend to one side, and if continually molested in this manner, it will uproot itself in pardonable dudgeon and swim away. That is to say it has several ways of reacting to the stimulus and seeking equilibrium, and, if one fails, it tries another. But now when it anchors itself again, it seems to have learnt something, for if again touched it does not go through the stages of contracting and of bending aside. It keeps to its more radical remedy and moves off again.’[83] It is doubtful whether there is any further complication of this type of behaviour among organisms which do not attain to the kind of behaviour that we have now to describe.

7. Herbert Spencer, as is well known, defined instinct as compound reflex action. Though this definition cannot, for various reasons, be accepted as adequate, it does suggest the essential nature of instinct, which consists in the response to a given but often vague stimulus of a more or less complicated series of reactions. Instinct is more than compound reflex action because it involves the organism as a whole, and is accompanied by, or is the outcome of, a mental process. All mental process is said to involve three aspects—the cognitive or the knowing of an object, the affective or feeling in regard to an object, and the conative or striving to or from an object—and these three aspects are to be found in all instinctive actions. The instinctive action is initiated by a sense-impression and is followed by results so important because the nervous system is innately organized to respond to certain sense-impressions. It is presumed that some kind of emotional excitement, however faint, always follows and that it gives rise to the striving that we see in the form of movement. Instinct has been defined as ‘an inherited or innate psychophysical disposition which determines its possessor to perceive and to pay attention to objects of a certain class, to experience an emotional excitement of a particular quality upon perceiving such an object, and to act in regard to it in a particular manner, or, at least, to experience an impulse to such action’.[84]

Every one is acquainted with many examples of instinctive action. ‘There are many instances of insects that invariably lay their eggs in the only place where the grubs, when hatched, will find the food they need and can eat, or where the larvae will be able to attach themselves as parasites to some host in a way that is necessary to their survival. In such cases it is clear that the behaviour of the parent is determined by the impression made on its senses by the appropriate objects or places: e. g. the smell of decaying fish leads the Carrion fly to deposit its eggs upon it; the sight or odour of some particular flower leads another to lay its eggs among the ovules of the flower, which serve as food to the grubs. Others go through more elaborate traits of action, as when the Mason-wasp lays its eggs in a mud nest, fills up the space with caterpillars, which it paralyses by means of well-directed stings, and seals it up; so that the caterpillars remain as a supply of fresh animal food for the young which the parent will never see and of whose needs it can have no knowledge or idea.’[85] To take some examples from among the Vertebrates, ‘pheasants, plovers, moor-hen, domestic chicks and ducklings, with many others, are active soon after birth, and exhibit powers of complex co-ordination, with little or no practice of the necessary limb movements. They walk and balance the body so soon and so well as to show that this mode of procedure is congenital, and has not to be gradually acquired through the guidance of experience. Young water birds swim with neat orderly strokes the first time they are gently placed in the water. Even little chicks a day or two old can swim well.’[86]

Enough has been said in the way of illustration, as numerous examples are familiar to every one. It is possible that instinctive behaviour may have to be attributed to so lowly a group of organisms as the flat worms. Instinct reaches its greatest development among the insects, and some examples have been given above of the amazingly intricate series of actions which are performed by insects, under the guidance of instinct. There have been two lines of mental evolution among animals, one culminating in the insects and the other in the Vertebrates. Among the former instincts have become very specialized; among the latter they have remained far more generalized. Among the latter again there has been a far higher development of intelligence than among the former, thus further distinguishing the two lines of mental evolution. It is probable, however, that intelligence, though certainly at times in a very primitive form, always accompanies instinct, and to the discussion of intelligence we must now turn.

8. We saw how as lowly an animal as an Infusorian can in a sense learn from experience. It is only when learning from experience reaches a more advanced stage that we speak of intelligent action. If we watch one of the higher animals which, under the influence of desire, is striving to satisfy this desire, we find that it behaves in the following manner. An animal, for instance, is shut up in a box with food outside. It is led by instinct to all kinds of sporadic activities; it will clutch and claw and make every kind of effort to extricate itself. If some simple catch has been contrived which opens the door and offers a way of escape, the animal will probably sooner or later accidentally operate the catch and escape. If the animal is replaced in the box many times, it is found to escape on the average sooner. It learns in fact in some degree to operate the catch and thus to gain freedom and food. The question which arises is what degree of mental development we have to assume in order to account for these facts.

The stage in learning from experience which follows next upon that present among some protozoa is exemplified by the chick which at first instinctively pecks at various objects. If it pecks at a yellow caterpillar with an unpleasant taste it will drop it. The next time, or after a series of such experiences, it will avoid the caterpillar. The explanation seems to be that a modified response arises directly from the sight of the caterpillar. The sense-impression has become charged with feeling that first arose as the result of experience. This simple explanation is to be preferred to that which would assume the realization by the chick of the relation between the position when it again finds itself with a yellow caterpillar before it and the nastiness which it previously experienced. The process is thus one of the revival of acquired meaning, and we have now to ask if a higher stage of mental process is ever to be attributed to animals or whether the behaviour of the animal in the cage is to be explained on the same lines as the behaviour of the chick.

‘Let us suppose’, says Professor Hobhouse, ‘revival to operate in a mind capable of perceiving three objects A B C in definite space and time relations, C being something desirable, e. g. food. If the three objects are present to the senses, the first two leading up to the third (e. g. as intervening objects in space), conation will be definitely directed to C via A and B. Let this have happened and then let A alone be given. If the animal is hungry, i.e. if there is a conational basis to go upon, A will, according to the law of revival, excite a conation corresponding to the previous one, but this was a conation definitely directed to B and C in succession as things standing in a definite relation to A. The animal then directs its efforts to a point where, in accordance with the first experience, B and C should be. It looks for them, or if B is some change which brings C about, sets itself to perform B and so obtain C. Its action is directed to something not given, and this appears to be the germ of a conation or practical idea.’[87] It is thus possible that in the behaviour of the animal in the box we may have to recognize the first step towards a higher mental process. Effort may be directed to something not given and thus there may be the first sign of the emergence of an idea. Whether this is so or not, such an idea is certainly not a general idea; it is merely a reference to something to come, and that is all. This is the highest degree of mental development that we can attribute to animals and it may be noticed that, this being so, there can probably be no true memory among animals. Explicit ideas, therefore, among animals, so far as they exist at all, do not give rise to other ideas following one another in sequence. They are isolated and serve merely to guide action.

9. The highest form of mental process attributable to animals reaches a fuller development among men. This stage of mental development has been called the stage of perceptual correlation. How far the apprehensions of direct relationships in consciousness are developed among animals is doubtful; there is no doubt that among men such relations are apprehended. Action, therefore, is not merely connected indirectly with the result, as in the example of the chick; action is undertaken with an end in view. If the chick came to apprehend the relation between the caterpillar and the unpleasant taste, it would have reached the fully developed stage of perceptual correlation; we have seen that we have to assume in this case a simpler state of mental process, though in certain cases a study of animal behaviour does suggest some approach to the higher stage. At this stage, which is fully developed only in man, the world ceases to be presented merely as sense-impressions charged with feeling and takes the shape of a mass of objects of perception related together and underlying the sense-impressions and the feelings evoked by them.

In man there is developed a still higher stage of mental process which is his peculiar possession and chief distinguishing characteristic. This is the stage of conceptual thought. In the perceptual stage activity is guided solely by the presence of the objects perceived. If there is any anticipation of the end, the action from moment to moment is still always guided by what is actually given. In the conceptual stage action is guided by an ideal anticipation of the end. What underlies mental process at this stage is generalization. The situation as given is broken up and analysed; elements common to it and to previous situations are recognized and synthesized. These two processes of analysis and synthesis go on side by side and concepts are formed which are outside the world of perception. Common elements in the perceptual order are recognized and there is thus made possible a grasp of the continuity running through experience.

Man is thus no longer guided by what is immediately given in experience; he can make plans and shape his actions with an ideal end in view. With the development of conceptual thought goes the development of language, whereby man learns from others and passes on to others what is in his mind. Of the stage of conceptual thought there will be more to say in a later chapter. This further development is only mentioned here in order to contrast mental process in its highest form in man with the process in animals.

10. We have now to consider the bearing of mental development upon the process of reproduction. All animals are endowed with a certain power of reproduction which we shall call fecundity. Fecundity is measured by the number of ripe ova produced, the number of spermatozoa having no direct bearing on fecundity. We have seen that the highest animals—those most nearly related to man—are gifted with instinct and intelligence. The value of intelligence lies in the fact that it enables instinct to adapt itself to the special circumstances of the moment and thus to bring about its end more surely. The sexual instinct is in this manner assisted by intelligence, and among animals which copulate the power of reproduction is thus able to realize itself to the full or almost to the full. There may be certain failures to achieve reproduction and certain cases of perverted instinct; to some of these cases we have already referred. Broadly speaking, however, it is true that the highest development in animals of instinct and intelligence works towards the fulfilment of that degree of fecundity which is innately given. It follows, therefore, that in this sense mental development among animals has not in any fashion changed the position of the higher animals when compared with that of the lower animals. Reproduction in the sea-urchin and reproduction among the mammals—vastly as the mental processes differ—is still similar in this respect. The power of reproduction is realized to the full or almost to the full. All that instinct and intelligence do is to ensure that in this more complex process of fertilization the full power of reproduction is as nearly as possible realized—that is to say, that nearly all ova are fertilized.

Among men, even in the lowest stage in which they have been studied, the position is entirely different. Owing to the development of conceptual thought men act with some ideal object in view. Customs grow up which in their origin must be traced to some process of reasoning, however obscure, and action deliberately undertaken, as well as custom, may affect the realization of the power of reproduction. Thus among the lowest of primitive races we find that men abstain from intercourse for various motives which we must regard as due to the presence of reason. Or again, they may practise certain forms of mutilation of the sexual organs which may affect the power of reproduction. The origin of such a custom may be hidden; it may be almost certain that it was not originated with any understanding of its effect upon reproduction and even that its effect has never been recognized; nevertheless originally such a custom could only have arisen if reason was present. Similarly, among the lowest races there are abundant examples of the practices of abortion and infanticide which, though they do not affect fecundity, have an important bearing upon the quantitative aspect of the population problem and are again the products of reason.

Among men, therefore, owing to the development of a higher stage of mental power, fecundity is not realized to the full, and we have to distinguish between the power of reproduction, which we have called fecundity, and the actual degree of reproduction, which we shall call fertility. It may, perhaps, assist to emphasize what is meant if for a moment we think of the reproductive process among animals as ‘mechanical’. The introduction of this term should not be taken to have any ultimate significance—any reference whatever to the true nature of mental process. It is only used as a convenient term to illustrate the difference between reproduction among animals and among men. Among all species in a state of nature reproduction may be thought of as ‘mechanical’, whereas reproduction among men is never ‘mechanical’. The number of young produced in the case of the human species is far from being completely correlated with the fecundity. There may be all degrees of difference between fecundity and fertility. Among species in a state of nature fecundity and fertility are for all practical purposes one and the same thing because reproduction is ‘mechanical’ whatever may be the stage which mental development has reached. Further, such differences as exist among species in a state of nature between fecundity—the number of ripe ova—and fertility—the number of fertilized ova—are due to failures of the gametes to meet and may be called, just as the whole process may be called, ‘mechanical’. Such ‘mechanical’ differences between fecundity and fertility may also be found in man but the chief cause of the differences in the case of man is altogether of another kind and is due, as we have seen, directly to the development of conceptual thought.

It was said above that it was proposed to show that certain generalizations can be made regarding the process of reproduction among species in a state of nature. This then is the first generalization. (Fecundity and fertility are closely correlated and, compared with the position among men, reproduction may be thought of as ‘mechanical’ and even the failures to realize the full power of fecundity may be thought of as ‘mechanical’.)

11. It has been mentioned that fecundity is very large among all species in a state of nature and we have now to ask what it is that determines how large it shall be. As we have seen, the fecundity is roughly measured by the number of ova produced, and this number clearly depends upon many factors, such as the beginning and duration of the mature period, the number of eggs produced at any one time, and the length of the period between the epochs of egg production. Into details of the proximate causes of the differences in fecundity it is not necessary to go. What it is desired to know is what factor or factors in general ultimately determine the strength of fecundity in each species. We may first take some examples of the degree of fecundity drawn from various groups.

The common whelk lays its eggs in capsules of which a great number are produced. It has been calculated that a small clump of such capsules of about two cubic inches in size contains about 200,000 eggs. Another mollusc, Aplysia, may lay from 2,000,000 to 3,000,000 at a time. ‘An oyster may have sixty million eggs and the average American yield is sixteen millions.’[88] The number of eggs found attached to the edible crab in the breeding season varies between half a million and three millions. A single pair of flies may produce 20,000 larvae. The number of eggs produced by parasites may very largely exceed these numbers. Among the vertebrates the fish are the most prolific class. ‘In a Ling 61 inches long and weighing 54 pounds the ovaries contained 28,361,000 eggs; a cod of 21½ pounds 6,652,000. The least prolific of British food fishes is the herring, in which the number of ovarian eggs varied from 21,000 to 47,000 in four specimens examined.’[89]

These vast numbers of eggs produced at any one time make theoretically possible a prodigious rate of increase. It has been calculated that a single cholera bacillus can give rise to sixteen hundred trillion of bacilli in a day, forming a solid mass weighing a hundred tons. ‘Wallace quotes Kerner to the effect that a common British weed (_Sisymbrium sophia_) often has three-quarters of a million seeds; if all grew to maturity for only three years the whole of the land surface of the globe would not hold them. An annual plant with only two seeds would be represented by 1,048,576 in the twenty-first year.... If all the progeny of one oyster survived and multiplied, its great-great-grand-children would number thirty-six with thirty-three noughts after it, and the heap of the shells would be eight times the size of the world. Huxley calculated that if the descendants of a single green-fly all survived and multiplied they would, at the end of summer, weigh down the population of China. The common house-fly lays eggs in batches of 120 to 150 at a time, and may lay five or six of these batches during its life of about three weeks in very hot weather. At the end of summer, if all developed, and if there were six generations, the progeny of a single pair, pressed together into a solid mass, would occupy a space of something like a quarter of a million cubic feet, allowing 200,000 flies to a cubic foot.’[90] ‘There is no exception’, says Darwin, in a well-known passage, ‘to the rule that every organic being naturally increases at so high a rate, that, if not destroyed, the earth would soon be covered by the progeny of a single pair. Even slow-breeding man has doubled in twenty years, and at this rate in less than a thousand years there would literally not be standing room for his progeny. Linnaeus has calculated that if an annual plant produced only two seeds—and there is no plant so unproductive as this—and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when thirty years old, and goes on breeding until ninety years old, bringing forth six young in the interval, and surviving till 100 years old; if this be so, after a period of 740 to 750 years there would be nearly ninety million elephants alive, descended from the first pair.’[91]

12. These examples show that the power of fecundity, which is always huge, is in many instances much greater than in others. Were it not for the fact that normally all but a small proportion of eggs are always fertilized, it might be suggested that in those cases in which there was no copulation a much larger number of eggs was necessary than among the higher forms, in order that a sufficient number should be fertilized. This, however, can only be a partial explanation of the larger number of eggs among those lower forms where there is no copulation.

In order to obtain an answer to the question as to what it is which determines the fecundity of any species, it is necessary to look into certain features of the life of animals and plants in a state of nature. Observation and deduction bring one remarkable fact to light. The number of adults of any species at any one season of the year, when compared with the number in the corresponding period in other years, remains upon the whole constant. This fact cannot be based upon statistics, for we cannot take anything approaching to a census. Nevertheless, it is an unavoidable deduction from the known facts. The more emphasis that is laid upon variations in numbers from season to season, the more apparent does it become that such differences are trivial when compared with the possible rate of increase. But we know that all but a small proportion of eggs are fertilized, and as we date the existence of a new member of the species from the moment of fertilization, it is clear that the numbers composing every new generation greatly exceed the number of adults to which the new generation owes its existence. It follows, therefore, that all but a small proportion of the young of each generation perish before the adult stage is reached. The most remarkable increases in the adults of any species ever recorded are negligible compared with the possible increase, and observation shows that as a general rule there is no increase at all.

Fecundity is, therefore, in some manner connected with this fact that the great majority of fertilized eggs do not give rise to adults, and in order to throw further light upon this connexion we must ask how it is that the young perish.

13. To make clear how it is that the young of every species perish on so large a scale, it is necessary to refer to the interdependence of all living organisms. This can perhaps best be illustrated by reference to the chief distinction between animals and plants. A difference in the mode of nutrition is that which chiefly distinguishes animals from plants. There are other differences but their importance is small compared with that we may now describe. The need of food is common to all living things and is due to the nature of the living substance called protoplasm—the physical basis of all life. Protoplasm is of a very complex constitution. It is for ever wasting away and if life is to be preserved food must be supplied to compensate for the loss. The need is as great among plants as it is among animals, but the means of supplying it are fundamentally different.

Plants feed upon very simple substances—salts of nitric acid, salts of ammonia, and carbonic acid. Among green plants carbonic acid is taken in from the air through small apertures in the leaves known as stomata; the salts of nitric acid and ammonia are absorbed from solution in the water of the soil. Carbonic acid and water are synthesized within the cells of the plant into starch; starch is converted into sugar and the sugar is combined with the salts of nitric acid and ammonia to form amino-acids which are eventually transformed into proteids. Thus the plant makes good the unavoidable waste of its living substance by elaborating the highly complex protoplasm from the simplest elements.

The method pursued by animals is entirely different. They feed upon complicated substances, which may be divided into proteids, fats, and carbo-hydrates. These substances do not take their places directly in the living cells of the body; they first undergo a process of digestion, after which they are assimilated. Digestion involves the breaking down of these complex foods to a certain stage; proteids, for example, are reduced to amino-acids, and starches to sugars. In these forms they are soluble and are taken up by the walls of the alimentary canal and are afterwards resynthesized into proteids and starches.

The importance of this distinction lies in the fact that the only method of obtaining the highly complex substances necessary for animals is to feed upon the tissues of other animals or plants. It is obvious that every animal species cannot feed upon some other animal species; in the end animals as a whole must depend upon plants because plants alone are able to elaborate the substances which animals need. In a sense, therefore, animals are parasitic upon plants; in any case the existence of animals is bound up with the continued existence of plants. This interdependence of living organisms runs all through the conditions under which species in a state of nature live and takes a variety of shapes. The dependence of one organism upon another is largely connected with the question of the provision of appropriate surroundings which are often only found in the proximity of certain other species. Many species can only flourish in the neighbourhood of trees. The interdependence between certain species is very intimate. There are many examples of what is known as symbiosis, as when a certain species of sea anemone lives on the back of a particular species of crab. Again, parasites, which alternate between one host and another, are dependent upon finding a member of a particular species at a certain time in their life-history, as otherwise they perish. Many examples of interdependence are within common knowledge, and, bearing this feature of organic life in mind, we may go on to ask how it is that, through the elimination of a great proportion of the young of every species, the number of adults remains upon the whole constant.

14. Taking animals first, it is probable that the most common cause of elimination lies in the fact that the young of all species are consumed by members of other species. It is difficult to estimate even roughly the relative importance of the various causes of elimination; ‘the causes’, says Darwin, ‘which check the natural tendency of each species to increase are most obscure’.[92] The particular factor mentioned, however, certainly takes a very prominent place. The young of marine and fresh-water animals almost all form food for other species and are obviously exposed to attack. So too, though perhaps not to so great a degree, are the young of terrestrial animals; whether we think of the larvae of insects or the eggs of birds, we find that they are in most cases liable to be consumed by enemies. Even where copulation is internal, the developing embryo is, except among mammals, seldom long retained within the body of the mother, and once exposed to the outside world it almost invariably becomes an object of prey to many enemies. In fact, wherever we look, this cause of elimination plays a very great part, arising immediately from that aspect of the interdependence of species which is derived from the mode among animals of making up for the wastage of protoplasm.

Elimination again is largely due to failure to find those conditions under which alone life can continue. These conditions may be connected with the nature of the organic or of the inorganic surroundings. When the young of the flat worm, known as the Liver Fluke, which infects sheep and causes a serious disease, passes from the sheep to the exterior, it can only maintain itself for a certain time in the free-living form. Unless within this time it meets with a certain species of snail into which it penetrates, it will die. Besides such failures to find suitable organic surroundings, there may be failures to find suitable inorganic surroundings. The larvae of such species as the mussel, which require suitable surroundings to which to attach themselves in order that the adult form may develop, will perish if such surroundings are not available. There is a further class of factors connected with the inorganic surroundings which bring about elimination. They may be summed up under the heading of external circumstances. Variations in temperature, moisture and so on, when they pass beyond a certain limit, which is more or less clearly marked for each species, are followed by death. Under this heading comes also death from accident, as when animals perish from the violence of storms and in any similar fashion. It is of interest to note that starvation is seldom the primary cause of death. It may be a secondary result of abnormal external circumstances; extreme cold, though not affecting directly the members of one species, may be fatal to members of another species upon which the former feed. But when circumstances are normal, so far as observation goes, starvation is rare.

15. Among plants the same three groups of factors can be traced, though their relative importance is not the same as among animals. Elimination through consumption as food, for instance, by other species is not so important. To a large extent plants can serve as food for animals and survive. Nevertheless the young, especially in the form of seeds, are very subject to attack by animals; seeds are one of the principal forms of food for many animal species, and this form of elimination plays a large part among plants. Dependence upon suitable organic and inorganic surroundings plays much the same rôle as it does among animals. Some species can only flourish in the shade of trees and others in the open. Some species require one kind of soil and others another. The importance of external circumstances again is very similar.

There is another factor of a somewhat different nature which is of great importance. ‘With plants’, says Darwin, ‘there is a vast destruction of seeds, but, from some observations which I have made, it appears that the seedlings suffer most from germinating in ground already thickly stocked with other plants.’[93] In order that a seed may germinate, it must not only fall upon suitable soil but must find enough suitable soil unoccupied. Otherwise it will not germinate or will not develop into an adult. This is different from anything which happens among animals, and is clearly an approach to starvation. It is, however, better thought of as the result of the inability of any given area to support more than a given amount of life. The endowment of any area may be such as to render it incapable of supporting life at all, or it may be such as to render it capable of supporting any degree of life up to and beyond that for which there is space. In the sea the amount of nitrogen is the limiting factor, and the deficiency of nitrogen is such that the question of space does not arise.[94] In many parts of the world’s surface, however, the endowment is such that more plants could be supported than there is space for.

In this connexion it may be noted that the limitation of the surface of the earth is not a cause of elimination in the true sense. This is best seen if we imagine the surface to be extended. If to a continent already inhabited there is added an unoccupied area, there will, if the new area is generally of the same nature as that alone formerly existing, be a spreading of organic life over the new area. It will only be at the fringe of the occupied area that there will be any difference in the amount of elimination of plant life. Unless the occupied area is of very small extent, there will be no difference except within this narrow fringe. In the fringe there will be a lessening of elimination, other things being equal, because there will not be the same number of seeds which fail to grow into plants owing to the previous occupation of suitable soil, as in the more central parts of the area. When the new area has been entirely occupied, the same conditions as existed in the smaller area will exist throughout the larger area. Further, if the surface available for occupation were indefinite in extent, there would, except at the fringe, which would in this case be permanent, be no difference as regards the amount of elimination. All that has been said applies equally well if, instead of imagining these additions to take place after evolution had reached its present stage, we imagine evolution to have taken place from the beginning on an area of indefinite extent.

16. We have, therefore, some idea of the manner in which the young of animals and plants perish. Let us consider any animal species; we find that the young are faced with a large number of dangers. The new members of the species may be consumed by some enemy before they have developed beyond the stage of the fertilized egg or at any stage in their development. They may in general not meet with favourable organic and inorganic surroundings, or at some particular stage they may not meet with the environment necessary. At any time they may perish from unfavourable external circumstances. The position of young plants is similar, and in addition they may fail to find sufficient space in which to live.

The dangers which any species encounters remain both in kind and in degree fairly constant over a considerable period of time, and, unless in each generation a number of young survives at least equal to the number of adults in the generation to which it owes its birth, the species will decline. It follows that the power of reproduction must be such as to ensure that at least this number of young will survive. The power of reproduction in any species is therefore connected with the sum of all the dangers which the young of the species encounter. But it is not true that the greater the fecundity the better for the species. Reproduction over and above this degree would place the young in a less favourable position. Competition bringing no corresponding advantages would be increased, and starvation or injurious semi-starvation would result. Any considerable increase in the strength of fecundity beyond that which is essential could not therefore be beneficial.[95]