Kitabı oku: «Essays Upon Heredity and Kindred Biological Problems», sayfa 23
But as soon as an organ becomes useless, the continued selection of individuals in which it is best developed must cease, and a process which I have termed panmixia takes place. When this process is in operation, not only those individuals with the best-developed organs have the chance of reproducing themselves, but also those individuals in which the organs are less well-developed. Hence follows a mixture of all possible degrees of perfection, which must in the course of time result in the deterioration of the average development of the organ. Thus a species which has retired into dark caverns must necessarily come to gradually possess less developed powers of vision; for defects in the structure of the eyes, which occur in consequence of individual variability, are not eliminated by natural selection, but may be transmitted and fixed in the descendants194. This result is all the more likely to happen, inasmuch as other organs which are of importance for the life of the species will gain what the functionless organ loses in size and nutrition. As at each stage of retrogressive transformation individual fluctuations always occur, a continued decline from the original degree of development will inevitably, although very slowly, take place, until the last remnant finally disappears. How inconceivably slowly this process goes on is shown by the numerous cases of rudimentary organs: by the above-mentioned embryonic sixth finger of man, or by the hind limbs of whales buried beneath the surface of the body, or by their embryonic tooth-germs. I believe that the very slowness with which functionless organs gradually disappear, agrees much better with my theory than with the one which has been hitherto held. The result of the disuse of an organ is considerable, even in the course of a single individual life, and if only a small fraction of such a result were transmitted to the descendants, the organ would be necessarily reduced to a minimum, in a hundred or at any rate in a thousand generations. But how many millions of generations may have elapsed since e. g. the teeth of the whalebone whales became useless, and were replaced by whalebone! We do not know the actual number of years, but we know that the whole material of the tertiary rocks has been derived from the older strata, deposited in the sea, elevated, and has been itself largely removed by denudation, since that time.
Now if this theory as to the causes of deterioration in disused organs be correct, it follows that rudimentary organs can only occur in species with sexual reproduction, and that they cannot be formed in species which are exclusively reproduced by the parthenogenetic method: for, according to my theory, variability depends upon sexual reproduction, while the deterioration of an organ when disused, no less than its improvement when in use, depends upon variability. There are therefore two reasons which lead us to expect that organs which are no longer used will remain unreduced in species with asexual reproduction: first, because only a very slight degree of hereditary variability can be present, viz. such a degree as was transmitted from the time when sexual reproduction was first abandoned by the ancestors; and, secondly, because even these slight degrees of variability are not combined, or, in other words, because panmixia cannot occur.
And the facts seem to point in the direction required by the theory, for superfluous organs do not become rudimentary in parthenogenetic species. For example, as far as my experience goes, the receptaculum seminis does not deteriorate, although it is, of course, altogether functionless when parthenogenesis has become established. I do not attach much importance to the fact that the Psychids and Solenobias—(genera of Lepidoptera which Siebold and Leuckart have shown to include species with parthenogenetic reproduction)—still retain the complete female sexual apparatus, because colonies containing males still occasionally occur in these species. Although the majority of colonies are now purely female, the occasional appearance of males points to the fact that the unisexuality of the majority cannot have been of very long duration. The process of transformation of the species from a bisexual into a unisexual form, only composed of females, is obviously incomplete, and is still in process of development. The case is similar with several species of Cynipidae, which reproduce by the parthenogenetic method. In these cases the occurrence of a very small proportion of males is the general rule, and is not confined to single colonies. Thus Adler195 counted 7 males and 664 females in the common Cynips of the rose.
In some Ostracodes, on the other hand, the males appear to be entirely wanting: at least, I have tried in vain for years to discover them in any locality or at any time of the year196.
Cypris vidua and Cypris reptans are such species. Now, although the transformation of these formerly bisexual species into purely unisexual female species appears to be complete197, yet the females still possess a large, pear-shaped receptaculum seminis, with its long spirally twisted duct, which is surrounded by a thick glandular layer. This is the more remarkable as the apparatus is very complicated in the Ostracodes, and retrogressive changes could be therefore easily detected. Furthermore among insects, in the genus Chermes the receptaculum seminis of the females has also remained unreduced, although the males appear to be entirely wanting, or at least have never been found, in spite of the united efforts of several acute observers198. The case is quite different in species which retain both sexual and parthenogenetic reproduction. Thus, the summer females of the Aphidae have lost the receptaculum seminis; and in these insects sexual reproduction has not ceased, but alternates regularly with parthenogenetic reproduction.
Certainly this proof of the truth of my theory as to the significance of sexual reproduction is far from settling the question: it only renders the theory highly probable. At present it is impossible to do more than this, because we do not yet possess a sufficient number of facts, for many of them could not have been sought for until after the theory had been suggested. We are here concerned with complicated phenomena, into which we cannot acquire an immediate insight, but can only attain it gradually.
But, nevertheless, I hope to have shown that the theory of natural selection is by no means incompatible with the theory of ‘the continuity of the germ-plasm;’ and, further, that if we accept this latter theory, sexual reproduction appears in an entirely new light: it has received a meaning, and has to a certain extent become intelligible.
The time in which men believed that science could be advanced by the mere collection of facts has long passed away: we know that it is not necessary to accumulate a vast number of miscellaneous facts, or to make as it were a catalogue of them; but we know that it is necessary to establish facts which, when grouped together in the light of a theory, will enable us to acquire a certain degree of insight into some natural phenomenon. In order to direct our attention to those new facts which are of immediate importance, it is absolutely necessary to seek the aid of some general theory for the arrangement and grouping of those which we already possess. This has been my object in the present paper.
But it may be perhaps objected that these phenomena are far too complicated to be attacked at the present time, and that we ought to wait quietly until the simpler phenomena have been resolved into their components. It may be asked whether the trouble and labour involved in the attempt to solve such questions as heredity or the transformation of species are not likely to be wasted and useless.
It is true that we sometimes meet with such opinions, but I believe that they are based upon a misunderstanding of the method which mankind has always followed in the investigation of nature, and which must therefore be founded upon the necessary relations existing between mankind and nature.
Science has often been compared to an edifice which has been solidly built by laying stone upon stone, until it has gradually risen to greater height and perfection. This comparison holds good up to a certain point, but it leads us to easily overlook the fact that this metaphorical building does not at any point rest upon the ground, and that, at least up to the present time, it has remained floating in the air. Not a single branch of science, not even Physics itself, has commenced building from below; all branches have begun to build at greater or less heights in the air, and have then built downwards: and even Physics has not yet reached the ground, for it is still very uncertain as to the nature of matter and force. In no single group of phenomena can we begin with the investigation of ultimate causes, because at this very point our means of reasoning stop short. We cannot begin with ultimate phenomena and gradually lead up to those which are more complicated: we cannot proceed synthetically and deductively, building up the phenomena from below; but we must as a rule proceed analytically and inductively, proceeding from above downwards.
No one will dispute these statements, but they are often forgotten, as is proved by the above-mentioned objection. If we were only permitted to attack the more complicated phenomena after gaining a complete insight into the simpler ones, then all scientists would be physicists and chemists, and not until Physics and Chemistry were done with should we be permitted to proceed to the investigation of organic nature. Under these circumstances we ought not to possess now any scientific theory of medicine; for the study of pathological physiology could not be commenced until normal physiology was completely known and understood. Yet how great a debt is owing by normal to pathological physiology! This is an example which enforces the conclusion that it is not only permissible, but in the highest degree advantageous, for the different spheres of phenomena to be attacked simultaneously.
Furthermore, if we had been compelled to proceed from the simple to the complex, what would have become of the Theory of Descent, the influence of which has advanced our knowledge of Biology to an altogether immeasurable extent?
But in this often repeated criticism that we are not yet ready to attack such complicated phenomena as heredity, is hidden still another fallacy, for it is implied that facts become less certain in proportion to the complexity of their causes. But is it less certain that the egg of an eagle developes into an eagle, or that the peculiarities of the father and mother are transmitted to the child, than that a stone falls to the ground when its support is taken away? Again, is it not possible to draw a perfectly distinct and certain conclusion as to the relative quantity of the material basis of heredity, present in the germ-cells of either parent, from the fact that the father and mother possess an equal or nearly equal share in heredity? But it is really unnecessary to argue in this way: why should we do more than re-affirm that such a method of procedure in scientific investigation is the only way by which we can gradually penetrate the hidden depths of natural phenomena?
No! Biology is not obliged to wait until Physics and Chemistry are completely finished; nor have we to wait for the investigation of the phenomena of heredity until the physiology of the cell is complete. Instead of comparing the progress of science to a building, I should prefer to compare it to a mining operation, undertaken in order to open up a freely branching lode. Such a lode must not be attacked from one point alone, but from many points simultaneously. From some of these we should quickly reach the deep-seated parts of the lode, from others we should only reach its superficial parts; but from every point some knowledge of the complex tout ensemble of the lode would be gained. And the more numerous the points of attack, the more complete would be the knowledge acquired, for valuable insight will be obtained in every place where the work is carried on with discretion and perseverance.
But discretion is indispensable for a fruitful result; or, leaving our metaphor, facts must be connected together by theories, if science is to advance. Just as theories are valueless without a firm basis of facts, so the mere collection of facts, without relation and without coherence, is utterly valueless. Science is impossible without hypotheses and theories: they are the plummets with which we test the depth of the ocean of unknown phenomena, and thus determine the future course to be pursued on our voyage of discovery. They do not give us absolute knowledge, but they afford us as much insight as it is possible for us to gain at the present time. To go on investigating without the guidance of theories, is like attempting to walk in a thick mist without a track and without a compass. We should get somewhere under these circumstances, but chance alone would determine whether we should reach a stony desert of unintelligible facts or a system of roads leading in some useful direction; and in most cases chance would decide against us.
In this sense I trust that the sign-post or compass which I offer may be accepted. Even though it should be its fate to be replaced by a better one at a later period, it will have fulfilled its object if it enables science to advance for even a short distance.
APPENDICES
Appendix I. Further considerations which oppose Nägeli’s
explanation of transformation as due to internal causes199
When I describe Nägeli’s theory of transformation as due to active causes lying within the organism, as a phyletic force of transformation, I do not mean to imply that it is one of those mysterious principles which, according to some writers, constitute the unconscious cause which directs the transformation of species. Nägeli’s idioplasm, which changes from within itself, is conceived as a thoroughly scientific, mechanically operating principle. This cause is undoubtedly capable of theoretical conception: the only question is whether it has any real existence. According to Nägeli, the growing organic substance, the idioplasm, not only represents a perpetuum mobile rendered possible as long as its substance continually receives from without the matter and force which are necessary for continuous growth, but it also represents a perpetuum variabile due to the action of internal causes200. But this is just the doubtful point, viz., whether the structure of the idioplasm itself compels it to change gradually during the course of its growth, or whether it is not rather the external conditions which compel the ever slightly varying idioplasm to change in a certain direction by the summation of small differences. It has been shown above that we do not gain anything by adopting Nägeli’s theory, because the main problem which organic nature offers for our solution, viz. adaptation, remains unsolved. Hence this theory does not explain the phenomena of nature, and I believe that there are also certain facts which are directly antagonistic to it.
If the idioplasm really possessed the power of spontaneous variability ascribed to it by Nägeli; if, as a result of its own growth, it were compelled to undergo gradual changes, and thus to produce new species, we should expect that the duration of species, genera, orders, &c. would be of approximately equal length respectively, at least in forms of equal structural complexity. The time required by the idioplasm to undergo such changes as would constitute transformation into a new species ought to be always the same at equal heights in the scale of organization, that is, with equal complexity in the molecular structure of the idioplasm. It appears to me to be a necessary consequence of Nägeli’s theory that the causes of transformation lie solely in this molecular structure of the idioplasm. If nothing more than a certain amount of growth, and consequently a certain period of time during which the organism reproduces itself with a certain intensity, is required to produce a change in the idioplasm, then we must conclude that the alteration in the latter must take place when this certain amount of growth has been reached, or after this certain period has elapsed. In other words, the time during which a species exists—from its origin as a modification of some older species, until its own transformation into a new one—must be the same in species with the same degree of organization. But the facts are very far from supporting this consequence of Nägeli’s theory. The duration of species is excessively variable: many arise and perish within the limits of a single geological formation, while others may be restricted to a very small part of a formation; others again may last through several formations. It must be admitted that we cannot estimate the exact position of extinct species in the scale of organization, and the differences may therefore depend upon differences of organization: or they may be explained by the supposition that certain species may have become incapable of transformation, and might, under favourable conditions, continue to exist for an indefinite period. But this reply would introduce a new hypothesis in direct antagonism to Nägeli’s theory, which assumes that the variability of idioplasm takes place as the consequence of mere growth, and necessarily depends upon molecular structure. Nägeli himself asserts that the essential substance (idioplasm) of the descendants of the earliest forms of life is in a state of perpetual change, which would continue even if the series of successive generations were indefinitely prolonged201. Hence there can be no rest in the process of change which the idioplasm must undergo; and this is as true of each single species as it is of the organic world taken as a whole. We could, perhaps, find shelter in the insufficiency of our geological knowledge, but the number of ascertained facts is too great for this to be possible. Thus it is well known that the genus Nautilus has lasted from Silurian times, through all the three geological periods, up to the present day: while all its Silurian allies (Orthoceras, Gomphoceras, Goniatites, &c.) became extinct at a comparatively early period.
A keen and clever controversialist might still bring forward many objections against such an argument. I do not therefore place too much dependence upon the geological facts by themselves, as a disproof of the self-variability of Nägeli’s idioplasm; for it must be admitted that the facts are not sufficiently complete for this purpose. For instance, in the case of Nautilus it might be argued that we do not know anything about the fossil Cephalopods of pre-Silurian times, and that it is therefore possible that the above-mentioned allies of Nautilus may have existed previously for as long a period as that through which Nautilus has lived in post-Silurian time. However this may be, it will be at least conceded that the geological facts do not lend any support to Nägeli’s theory, for we can see no trace of even an approximately regular succession of forms.
Appendix II. Nägeli’s explanation of adaptation202
In order to explain adaptation Nägeli assumes that, under certain circumstances, external influences may cause slight permanent changes in the idioplasm. If then such influences act continually in the same direction during long periods of time, the changes in the idioplasm may increase to a perceptible amount, i. e. to a degree which makes itself felt in visible external characters203. But such changes alone could not be considered as adaptations, for the essential character of an adaptation is that it must be a purposeful change. Nägeli, however, brings forward the fact that external stimuli often produce their chief effects at that very part of the organism to which the stimuli themselves were applied. ‘If the results are detrimental, the organism attempts to defend itself against the stimulus: a confluence of nutrient fluid takes place towards the part upon which the stimulus has acted, and new tissues are formed which restore the integrity of the organism by replacing the lost structures as far as possible. Thus in plants the healthy tissues begin to grow actively around the seat of an injury, tending to close it up, and to afford protection by impenetrable layers of cork.’ Purposeful reactions of this kind are certainly common in the organic world, occurring in animals as well as in plants. Thus in the human body an injury causes a rapid growth of the surrounding tissues, which leads to the closing-up of the wound; while in the Salamander even the amputated leg or tail is replaced by growth. An extreme example of these purposeful reactions is afforded by the tree-frog (Hyla), which is of a light-green colour when seated upon a light-green leaf, but becomes dark brown when transferred to dark surroundings. Hence this animal adapts itself to the colour of its environment, and thus gains protection from its enemies.
Admitting this capability on the part of organisms to react under certain stimuli in a purposeful manner, the question remains whether such a power is a primitive original quality belonging to the essential nature of each organism. The power of changing the colour of the skin in correspondence with that of the surroundings is not very common in the animal kingdom. In the frog this power depends upon a highly complex reflex mechanism. Certain chromatophores in the skin are connected with nerves204 which pass to the brain and are there brought into relation, by means of nerve-cells, with the nervous centres of the organ of vision. The relation is of such a kind that strong light falling upon the retina constitutes a stimulus for the production of an impulse, which is conducted, along the previously mentioned motor nerves, from the brain to the chromatophores, thus determining the contraction of these latter and the consequent appearance of a light-coloured skin. When the strong stimulus (of light) ceases, the chromatophores expand again, and the skin becomes dark. That the chromatophores do not themselves react upon the direct stimulus of light was proved by Lister205, who showed that blind frogs do not possess the power of altering their colour in correspondence with that of their environment. It is quite obvious that in this case we are not dealing with a primary, but with a secondarily produced character; and it has yet to be proved that all the purposeful reactions mentioned by Nägeli are not similarly secondary characters or adaptations, and thus very far from being primitive qualities of the organic substance of the forms in which they occur.
I do not by any means doubt that some of the reactions witnessed in organisms do not depend upon adaptation, but such reactions are not usually purposeful. Curiously enough, Nägeli mentions the formation of galls in plants among his instances of purposeful reactions under external stimuli. I think, however, that it can hardly be maintained that the galls are of any use to the plant: on the contrary, they may even be very injurious to it. The gall is only useful to the insect which it protects and supplies with food. The recent and most excellent investigations of Adler206 and of Beyerinck207 have shown that the puncture made by the Cynips in depositing its eggs is not the stimulus which produces the gall, as was formerly believed to be the case, but that such a stimulus is provided by the larva which developes from the egg. The presence of this small, actively moving, foreign body stimulates the tissue of the plant in a definite manner, always producing a result which is advantageous to the larva and not to the plant. It would be to the advantage of the latter if it killed the intruding larva, either enclosing it by woody tissue devoid of nourishment, or poisoning it by some acrid secretion, or simply crushing it by the active growth of the surrounding tissues. But nothing of the kind occurs: in fact an active growth of cells (forming the so-called ‘Blastem’ of Beyerinck) takes place around the embryo, while it is still enclosed in the egg-capsule; but the growth is not such as to crush the embryo, which remains free in the cavity, the so-called larval chamber, which is formed around it. It would be out of place to discuss here the question as to how we can conceive that the plant is thus compelled to produce a growth which is at any rate indifferent and may be injurious to it; and which, moreover, is exactly adapted to the needs of its insect-enemy. But it is at all events obvious that this cannot be an example of a self-protecting reaction under a stimulus, and that therefore an organism does not always respond to external stimuli in a manner useful to itself.
But even if we could accept the suggestion that the purposeful reaction of an organism under stimulation is a primary and not a secondarily produced character, such a principle would by no means suffice for the explanation of existing adaptations. Nägeli attempts to explain certain selected cases of adaptation as the direct results of external stimuli. He looks upon the thick hairy coat of mammals in arctic regions, and the winter covering of animals in temperate regions, as a direct reaction of the skin under the influence of cold. He considers that the horns, claws, and tusks of animals have arisen directly as reactions under stimuli applied to certain parts of the surface of the body in attack and defence208. This interpretation is similar to that offered by Lamarck at the beginning of this century. At first sight such a suggestion appears to be plausible, for the acquisition of a thick hairy covering by the mammals of temperate regions is actually contemporaneous with the cold season of the year. But the question arises as to whether the production of a larger number of hairs at the beginning of winter is not merely another instance of a secondary character, like the assumption of a green colour by the tree-frog under the stimulus exerted by strong light.
In the case of the hairy coat it is only necessary to produce a larger number of structures such as had existed previously; but how can it have been possible for the petals of flowers, with their peculiar and complex forms, to have been developed from stamens as a direct result of the insects which visit them in order to obtain pollen and nectar? How could the creeping of these insects and the small punctures made by them constitute stimuli for the production of an increased rate of growth? And how is it possible in any way to explain, by mere increase in growth, the origin of a structure in which each part has its own distinct meaning and plays a peculiar part in attracting insects and in the process of cross-fertilization effected by them? Even if the manifold peculiarities of form could be explained in this way, how can such an explanation possibly hold for the colours of flowers? How could the white colour of flowers which open at night be explained as the direct result of the creeping of insects? How can the suggestion of such a cause offer any interpretation of the fact that flowers which open by day are tinted with various colours, or of the fact that there is often a bright or highly coloured spot which shows the way to the hidden nectary?
There are, moreover, a large number of very striking adaptations in form and colour, for which no stimulus acting directly upon the organism can be found. Can we imagine that the green caterpillar209, plant-bug, or grasshopper, sitting among green surroundings, is thus exposed to a stimulus which directly produces the green colour in the skin? Can the walking-stick insect, which resembles a brown twig, be subject to a transforming stimulus by sitting on such branches or by looking at them? Or again, if we consider the phenomena of mimicry, how can one species of butterfly, by flying about with another species, exercise upon the latter such an influence as to render it similar to the first in appearance? In many cases of mimicry, the mimicked and the mimicking species do not even live in the same place, as we see in the moths, flies, and beetles which resemble in appearance the much-dreaded wasps.
The interpretation of adaptation is the weak part of Nägeli’s theory, and it is somewhat remarkable that so acute a thinker should not have perceived this himself. One very nearly gains the impression that Nägeli does not wish to understand the theory of natural selection. He says, for instance, in speaking of the mutual adaptation observable between the proboscis, the so-called ‘tongue’ of butterflies, and flowers with tubular corolla210:—‘Among the most remarkable and commonest adaptations observable in the forms of flowers, are the corollas with long tubes considered in relation to the long “tongues” of insects, which suck the nectar from the bottom of the long narrow tubes, and at the same time effect the cross-fertilization of the plant. Both these arrangements have been gradually developed to their present degree of complexity—the long-tubed corollas from those without tubes, and from those with short ones, the long “tongues” from short ones. Undoubtedly both have been developed at the same rate so that the length of both sets of structures has always remained the same.’
No objection can be raised against these statements, but Nägeli goes on to say:—‘But how can such a process of development be explained by the theory of natural selection, for at each stage in the process the adaptation was invariably complete. The tube of the corolla and the “tongue” must have reached, for instance, at a certain time, a length of 5 or 10 mm. If now the tube of the corolla became longer in some plants, such an alteration would have been disadvantageous because the insects would be no longer able to obtain food from them, and would therefore visit flowers with shorter tubes. Hence, according to the theory of natural selection, the longer tubes ought to have disappeared. If on the other hand the “tongue” became longer in some insects, such a change would be superfluous and should have been given up, according to the same theory, as unnecessary structural waste. The simultaneous change in the two structures must, according to the theory of natural selection, be due to the same principle as that by which Münchhausen pulled himself out of a bog by means of his own pig-tail.’
