darwin and modern science-第18章
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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…butterflies； 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 timefor 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…butterfly； 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…butterflies； 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 butterfly 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 degree 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…butterflies； 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 associated 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 themthe 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。 （〃The Evolution Theory〃； London； 1904； page 144。）
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 outwhat has long been known； but is not universally admitted； even nowthat 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 transformationsthe 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 passages； 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 assume 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 usadaptation and hereditybut 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 passively 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。
Butit will be objectedthe substance of which the feather consists； this peculiar kind of horny 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 passively 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 class； but which was continued into whole Phyla of animals； with continual fresh adaptations to the special conditions of each species； family； or class； 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。 （〃Variation under Domestication〃； 1875 II。 pages 426； 427。） 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。 （The Author and Editor are indebted to Professor Poulton for kindly assisting in the revision of the proof of this Essay。）
IV。 VARIATION。
By HUGO DE VRIES； Professor of Botany in the University of Amsterdam。
I。 DIFFERENT KINDS OF VARIABILITY。
Before Darwin； little was known concerning the phenomena of
