experiments with young birds proved that this is really the case, and at the same time furnished an additional argument against the _Lamarckian principle_.
In addition to the mimicry-rings first observed in South America, others have been described from Tropical India by Moore, and by Poulton and Dixey from Africa, and we may expect to learn many more interesting facts in this connection. Here again the preliminary postulates of the theory are satisfied. And how much more that would lead to the same conclusion might be added!
As in the case of mimicry many species have come to resemble one another through processes of selection, so we know whole cla.s.ses of phenomena in which plants and animals have become adapted to one another, and have thus been modified to a considerable degree. I refer particularly to the relation between flowers and insects. Darwin has shown that the originally inconspicuous blossoms of the phanerogams were transformed into flowers through the visits of insects, and that, conversely, several large orders of insects have been gradually modified by their a.s.sociation with flowers, especially as regards the parts of their body actively concerned. Bees and b.u.t.terflies in particular have become what they are through their relation to flowers. In this case again all that is apparently contradictory to the theory can, on closer investigation, be beautifully interpreted in corroboration of it. Selection can give rise only to what is of use to the organism actually concerned, never to what is of use to some other organism, and we must therefore expect to find that in flowers only characters of use to _themselves_ have arisen, never characters which are of use to insects only, and conversely that in the insects characters useful to them and not merely to the plants would have originated. For a long time it seemed as if an exception to this rule existed in the case of the fertilisation of the yucca blossoms by a little moth, _p.r.o.nuba yuccasella_. This little moth has a sickle-shaped appendage to its mouth-parts which occurs hi no other Lepidopteron, and which is used for pushing the yellow pollen into the opening of the pistil, thus fertilising the flower. Thus it appears as if a new structure, which is useful only to the plant, has arisen in the insect. But the difficulty is solved as soon as we learn that the moth lays its eggs in the fruit-buds of the Yucca, and that the larvae, when they emerge, feed on the developing seeds. In effecting the fertilisation of the flower the moth is at the same time making provision for its own offspring, since it is only after fertilisation that the seeds begin to develop. There is thus nothing to prevent our referring this structural adaptation in _p.r.o.nuba yuccasella_ to processes of selection, which have gradually transformed the maxillary palps of the female into the sickle-shaped instrument for collecting the pollen, and which have at the same time developed in the insect the instinct to press the pollen into the pistil.
In this domain, then, the theory of selection finds nothing but corroboration, and it would be impossible to subst.i.tute for it any other explanation, which now that the facts are so well known, could be regarded as a serious rival to it. That selection is a factor, and a very powerful factor in the evolution of organisms, can no longer be doubted. Even although we cannot bring forward formal proofs of it _in detail_, cannot calculate definitely the size of the variations which present themselves, and their selection-value, cannot, in short, reduce the whole process to a mathematical formula, yet we must a.s.sume selection, because it is the only possible explanation applicable to whole cla.s.ses of phenomena, and because, on the other hand, it is made up of factors which we know can be proved actually to exist, and which, _if_ they exist, must of logical necessity cooperate in the manner required by the theory. _We must accept it because the phenomena of evolution and adaptation must have a natural basis, and because it is the only possible explanation of them._[53]
Many people are willing to admit that selection explains adaptations, but they maintain that only a part of the phenomena are thus explained, because everything does not depend upon adaptation. They regard adaptation as, so to speak, a special effort on the part of Nature, which she keeps in readiness to meet particularly difficult claims of the external world on organisms. But if we look at the matter more carefully we shall find that adaptations are by no means exceptional, but that they are present everywhere in such enormous numbers, that it would be difficult in regard to any structure whatever, to prove that adaptation had _not_ played a part in its evolution.
How often has the senseless objection been urged against selection that it can create nothing, it can only reject. It is true that it cannot create either the living substance or the variations of it; both must be given. But in rejecting one thing it preserves another, intensifies it, combines it, and in this way _creates_ what is new.
_Everything_ in organisms depends on adaptation; that is to say, everything must be admitted through the narrow door of selection, otherwise it can take no part in the building up of the whole. But, it is asked, what of the direct effect of external conditions, temperature, nutrition, climate and the like? Undoubtedly these can give rise to variations, but they too must pa.s.s through the door of selection, and if they cannot do this they are rejected, eliminated from the const.i.tution of the species.
It may, perhaps, be objected that such external influences are often of a compelling power, and that every animal must submit to them, and that thus selection has no choice and can neither select nor reject.
There may be such cases; let us a.s.sume for instance that the effect of the cold of the Arctic regions was to make all the mammals become black; the result would be that they would all be eliminated by selection, and that no mammals would be able to live there at all. But in most cases a certain percentage of animals resists these strong influences, and thus selection secures a foothold on which to work, eliminating the unfavourable variation, and establishing a useful colouring, consistent with what is required for the maintenance of the species.
Everything depends upon adaptation! We have spoken much of adaptation in colouring, in connection with the examples brought into prominence by Darwin, because these are conspicuous, easily verified, and at the same time convincing for the theory of selection. But is it only desert and polar animals whose colouring is determined through adaptation? Or the leaf-b.u.t.terflies, and the mimetic species, or the terrifying markings, and "warning-colours" and a thousand other kinds of sympathetic colouring? It is, indeed, never the colouring alone which makes up the adaptation; the structure of the animal plays a part, often a very essential part, in the protective disguise, and thus _many_ variations may cooperate towards _one_ common end. And it is to be noted that it is by no means only external parts that are changed; internal parts are _always_ modified at the same time--for instance, the delicate elements of the nervous system on which depend the _instinct_ of the insect to hold its wings, when at rest, in a perfectly definite position, which, in the leaf-b.u.t.terfly, has the effect of bringing the two pieces on which the marking occurs on the anterior and posterior wing into the same direction, and thus displaying as a whole the fine curve of the midrib on the seeming leaf. But the wing-holding instinct is not regulated in the same way in all leaf-b.u.t.terflies; even our indigenous species of Vanessa, with their protective ground-colouring, have quite a distinctive way of holding their wings so that the greater part of the anterior wing is covered by the posterior when the b.u.t.terfly is at rest. But the protective colouring appears on the posterior wing and on the tip of the anterior, _to precisely the distance to which it is left uncovered_. This occurs, as Standfuss has shown, in different degrees in our two most nearly allied species, the uncovered portion being smaller in _V. urticae_ than in _V. polychloros_. In this case, as in most leaf-b.u.t.terflies, the holding of the wing was probably the primary character; only after that was thoroughly established did the protective marking develop. In any case, the instinctive manner of holding the wings is a.s.sociated with the protective colouring, and must remain as it is if the latter is to be effective. How greatly instincts may change, that is to say, may be adapted, is shown by the case of the Noctuid "shark" moth, _Xylina vetusta_. This form bears a most deceptive resemblance to a piece of rotten wood, and the appearance is greatly increased by the modification of the innate impulse to flight common to so many animals, which has here been transformed into an almost contrary instinct. This moth does not fly away from danger, but "feigns death," that is, it draws antennae, legs and wings close to the body, and remains perfectly motionless. It may be touched, picked up, and thrown down again, and still it does not move. This remarkable instinct must surely have developed simultaneously with the wood-colouring; at all events, both cooperating variations are now present, and prove that both the external and the most minute internal structure have undergone a process of adaptation.
The case is the same with all structural variations of animal parts, which are not absolutely insignificant. When the insects acquired wings they must also have acquired the mechanism with which to move them--the musculature, and the nervous apparatus necessary for its automatic regulation. All instincts depend upon compound reflex mechanisms and are just as indispensable as the parts they have to set in motion, and all may have arisen through processes of selection if the reasons which I have elsewhere given for this view are correct.[54]
Thus there is no lack of adaptations within the organism, and particularly in its most important and complicated parts, so that we may say that there is no actively functional organ that has not undergone a process of adaptation relative to its function and the requirements of the organism. Not only is every gland structurally adapted, down to the very minutest histological details, to its function, but the function is equally minutely adapted to the needs of the body. Every cell in the mucous lining of the intestine is exactly regulated in its relation to the different nutritive substances, and behaves in quite a different way towards the fats, and towards nitrogenous substances, or peptones.
I have elsewhere called attention to the many adaptations of the whale to the surrounding medium, and have pointed out--what has long been known, but is not universally admitted, even now--that in it a great number of important organs have been transformed in adaptation to the peculiar conditions of aquatic life, although the ancestors of the whale must have lived, like other hair-covered mammals, on land. I cited a number of these transformations--the fish-like form of the body, the hairlessness of the skin, the transformation of the fore-limbs to fins, the disappearance of the hind-limbs and the development of a tail fin, the layer of blubber under the skin, which affords the protection from cold necessary to a warm-blooded animal, the disappearance of the ear-muscles and the auditory pa.s.sages, the displacement of the external nares to the forehead for the greater security of the breathing-hole during the brief appearance at the surface, and certain remarkable changes in the respiratory and circulatory organs which enable the animal to remain for a long time under water. I might have added many more, for the list of adaptations in the whale to aquatic life is by no means exhausted; they are found in the histological structure and in the minutest combinations in the nervous system. For it is obvious that a tail-fin must be used in quite a different way from a tail, which serves as a fly-brush in hoofed animals, or as an aid to springing in the kangaroo or as a climbing organ; it will require quite different reflex-mechanisms and nerve combinations in the motor centres.
I used this example in order to show how unnecessary it is to a.s.sume a special internal evolutionary power for the phylogenesis of species, for this whole order of whales is, so to speak, _made up of adaptations_; it deviates in many essential respects from the usual mammalian type, and all the deviations are adaptations to aquatic life. But if precisely the most essential features of the organisation thus depend upon adaptation, what is left for a phyletic force to do, since it is these essential features of the structure it would have to determine? There are few people now who believe in a phyletic evolutionary power, which is not made up of the forces known to us--adaptation and heredity--but the conviction that _every_ part of an organism depends upon adaptation has not yet gained a firm footing.
Nevertheless, I must continue to regard this conception as the correct one, as I have long done.
I may be permitted one more example. The feather of a bird is a marvellous structure, and no one will deny that as a whole it depends upon adaptation. But what part of it _does not_ depend upon adaptation? The hollow quill, the shaft with its hard, thin, light cortex, and the spongy substance within it, its square section compared with the round section of the quill, the flat barbs, their short, hooked barbules which, in the flight-feathers, hook into one another with just sufficient firmness to resist the pressure of the air at each wing-beat, the lightness and firmness of the whole apparatus, the elasticity of the vane, and so on. And yet all this belongs to an organ which is only pa.s.sively functional, and therefore can have nothing to do with the _Lamarckian principle_. Nor can the feather have arisen through some magical effect of temperature, moisture, electricity, or specific nutrition, and thus selection is again our only anchor of safety.
But--it will be objected--the substance of which the feather consists, this peculiar kind of h.o.r.n.y substance, did not first arise through selection in the course of the evolution of the birds, for it formed the covering of the scales of their reptilian ancestors. It is quite true that a similar substance covered the scales of the Reptiles, but why should it not have arisen among them through selection? Or in what other way could it have arisen, since scales are also pa.s.sively useful parts? It is true that if we are only to call adaptation what has been acquired by the species we happen to be considering, there would remain a great deal that could not be referred to selection; but we are postulating an evolution which has stretched back through aeons, and in the course of which innumerable adaptations took place, which had not merely ephemeral persistence in a genus, a family or a cla.s.s, but which was continued into whole Phyla of animals, with continual fresh adaptations to the special conditions of each species, family, or cla.s.s, yet with persistence of the fundamental elements. Thus the feather, once acquired, persisted in all birds, and the vertebral column, once gained by adaptation in the lowest forms, has persisted in all the Vertebrates from Amphioxus upwards, although with constant readaptation to the conditions of each particular group. Thus everything we can see in animals is adaptation, whether of to-day, or of yesterday, or of ages long gone by; every kind of cell, whether glandular, muscular, nervous, epidermic, or skeletal, is adapted to absolutely definite and specific functions, and every organ which is composed of these different kinds of cells contains them in the proper proportions, and in the particular arrangement which best serves the function of the organ; it is thus adapted to its function.
All parts of the organism are tuned to one another, that is, _they are adapted to one another_, and in the same way _the organism as a whole is adapted to the conditions of its life, and it is so at every stage of its evolution._
But all adaptations _can_ be referred to selection; the only point that remains doubtful is whether they all _must_ be referred to it.
However that may be, whether the _Lamarckian principle_ is a factor that has cooperated with selection in evolution, or whether it is altogether fallacious, the fact remains, that selection is the cause of a great part of the phyletic evolution of organisms on our earth.
Those who agree with me in rejecting the _Lamarckian principle_ will regard selection as the only _guiding_ factor in evolution, which creates what is new out of the transmissible variations, by ordering and arranging these, selecting them in relation to their number and size, as the architect does his building-stones so that a particular style must result.[55] But the building-stones themselves, the variations, have their basis in the influences which cause variation in those vital units which are handed on from one generation to another, whether, taken together they form the _whole_ organism, as in Bacteria and other low forms of life, or only a germ-substance, as in unicellular and multicellular organisms.
FOOTNOTES:
[Footnote 33: _Vortrage uber Descendenztheorie_, Jena, 1904, II. 269.
Eng. Transl. London, 1904, II. p. 317.]
[Footnote 34: See Poulton, _Essays on Evolution_, Oxford, 1908. pp.
xix-xxii.]
[Footnote 35: _Origin of Species_ (6th edit), pp. 176 _et seq._]
[Footnote 36: Chun, _Reise der Valdivia_, Leipzig, 1904.]
[Footnote 37: Plate, _Selektionsprinzip u. Probleme der Artbildung_ (3rd edit.), Leipzig, 1908.]
[Footnote 38: _Studien zur Descendenz-Theorie_ II., "Die Enstehung der Zeichnung bei den Schmetterlings-raupen," Leipzig, 1876.]
[Footnote 39: _Origin of Species_ (6th edit.), p. 232.]
[Footnote 40: _Origin of Species_, p. 233; see also edit. 1, p. 242.]
[Footnote 41: _Ibid._ p. 230.]
[Footnote 42: _The Effect of External Influences upon Development_, Romanes Lecture, Oxford, 1894.]
[Footnote 43: See Poulton, _Essays on Evolution_, 1908, pp. 316, 317.]
[Footnote 44: _The Evolution Theory_, London, 1904, I. p. 219.]
[Footnote 45: _Report of the British a.s.sociation_ (Bristol, 1898), London, 1899, pp. 906-909.]
[Footnote 46: _Proc. Ent. Soc._, London, May 6, 1903.]
[Footnote 47: _Essays on Evolution_, 1889-1907, Oxford, 1908, _pa.s.sim_, e.g. p. 269.]
[Footnote 48: The expression does not refer to all the enemies of this b.u.t.terfly; against ichneumon-flies, for instance, their unpleasant smell usually gives no protection.]
[Footnote 49: Professor Poulton has corrected some wrong descriptions which I had unfortunately overlooked in the Plates of my book _Vortrage uber Descendenztheorie_, and which refer to _Papilio darda.n.u.s_ (_merope_). These mistakes are of no importance as far as an understanding of the mimicry-theory is concerned, but I hope shortly to be able to correct them in a later edition.]
[Footnote 50: _Journ. Linn. Soc. London_ (_Zool._), Vol. xxvi. 1898, pp. 598-602.]
[Footnote 51: In _Kosmos_, 1879, p. 100.]
[Footnote 52: _Habit and Instinct_, London. 1896.]
[Footnote 53: This has been discussed in many of my earlier works. See for instance _The All-Sufficiency of Natural Selection, a reply to Herbert Spencer_, London, 1893.]
[Footnote 54: _The Evolution Theory_, London, 1904, p. 144.]
[Footnote 55: _Variation under Domestication_, 1875, II. pp. 426, 427.]
III
HEREDITY AND VARIATION IN MODERN LIGHTS
BY W. BATESON, M.A., F.R.S.
_Professor of Biology in the University of Cambridge_