按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
peratures； and we know other butterflies； as for instance our Garden Whites； in which the differences between the two generations are not nearly so marked； indeed； they are so little apparent that they are scarcely likely to be noticed except by experts。 Thus here again there are small initial steps； some of which； indeed； must be regarded as adaptations； such as the green…sprinkled or lightly tinted under…surface which gives them a deceptive resemblance to parsley or to Cardamine leaves。
Even if saltatory variations do occur； we cannot assume that these HAVE EVER LED TO FORMS WHICH ARE CAPABLE OF SURVIVAL UNDER THE CONDITIONS OF WILD LIFE。 Experience has shown that in plants which have suddenly varied the power of persistence is diminished。 Korschinksky attributes to them weaknesses of organisation in general； 〃they bloom late； ripen few of their seeds； and show great sensitiveness to cold。〃 These are not the characters which make for success in the struggle for existence。
We must briefly refer here to the viewsmuch discussed in the last decade of H。 de Vries； who believes that the roots of transformation must be sought for in SALTATORY VARIATIONS ARISING FROM INTERNAL CAUSES； and distinguishes such MUTATIONS； as he has called them； from ordinary individual variations； in that they breed true； that is； with strict inbreeding they are handed on pure to the next generation。 I have elsewhere endeavoured to point out the weaknesses of this theory （〃Vortrage uber Descendenztheorie〃； Jena； 1904； II。 269。 English Translation London； 1904； II。 page 317。）； and I am the less inclined to return to it here that it now appears （See Poulton； 〃Essays on Evolution〃； Oxford； 1908； pages xix…xxii。） that the far…reaching conclusions drawn by de Vries from his observations on the Evening Primrose； Oenothera lamarckiana； rest upon a very insecure foundation。 The plant from which de Vries saw numerous 〃species〃his 〃mutations〃arise was not； as he assumed； a WILD SPECIES that had been introduced to Europe from America； but was probably a hybrid form which was first discovered in the Jardin des Plantes in Paris； and which does not appear to exist anywhere in America as a wild species。
This gives a severe shock to the 〃Mutation theory；〃 for the other ACTUALLY WILD species with which de Vries experimented showed no 〃mutations〃 but yielded only negative results。
Thus we come to the conclusion that Darwin （〃Origin of Species〃 （6th edition）； pages 176 et seq。） was right in regarding transformations as taking place by minute steps； which； if useful； are augmented in the course of innumerable generations； because their possessors more frequently survive in the struggle for existence。
（b） SELECTION…VALUE OF THE INITIAL STEPS。
Is it possible that the significant deviations which we know as 〃individual variations〃 can form the beginning of a process of selection？ Can they decide which is to perish and which to survive？ To use a phrase of Romanes； can they have SELECTION…VALUE？
Darwin himself answered this question； and brought together many excellent examples to show that differences； apparently insignificant because very small； might be of decisive importance for the life of the possessor。 But it is by no means enough to bring forward cases of this kind； for the question is not merely whether finished adaptations have selection…value； but whether the first beginnings of these； and whether the small； I might almost say minimal increments； which have led up from these beginnings to the perfect adaptation； have also had selection…value。 To this question even one who； like myself； has been for many years a convinced adherent of the theory of selection； can only reply： WE MUST ASSUME SO； BUT WE CANNOT PROVE IT IN ANY CASE。 It is not upon demonstrative evidence that we rely when we champion the doctrine of selection as a scientific truth； we base our argument on quite other grounds。 Undoubtedly there are many apparently insignificant features； which can nevertheless be shown to be adaptations for instance； the thickness of the basin…shaped shell of the limpets that live among the breakers on the shore。 There can be no doubt that the thickness of these shells； combined with their flat form； protects the animals from the force of the waves breaking upon them；but how have they become so thick？ What proportion of thickness was sufficient to decide that of two variants of a limpet one should survive； the other be eliminated？ We can say nothing more than that we infer from the present state of the shell； that it must have varied in regard to differences in shell…thickness； and that these differences must have had selection…value； no proof therefore； but an assumption which we must show to be convincing。
For a long time the marvellously complex RADIATE and LATTICE…WORK skeletons of Radiolarians were regarded as a mere outflow of 〃Nature's infinite wealth of form；〃 as an instance of a purely morphological character with no biological significance。 But recent investigations have shown that these； too； have an adaptive significance （Hacker）。 The same thing has been shown by Schutt in regard to the lowly unicellular plants； the Peridineae； which abound alike on the surface of the ocean and in its depths。 It has been shown that the long skeletal processes which grow out from these organisms have significance not merely as a supporting skeleton； but also as an extension of the superficial area； which increases the contact with the water…particles； and prevents the floating organisms from sinking。 It has been established that the processes are considerably shorter in the colder layers of the ocean； and that they may be twelve times as long （Chun； 〃Reise der Valdivia〃； Leipzig； 1904。） in the warmer layers； thus corresponding to the greater or smaller amount of friction which takes place in the denser and less dense layers of the water。
The Peridineae of the warmer ocean layers have thus become long…rayed； those of the colder layers short…rayed； not through the direct effect of friction on the protoplasm； but through processes of selection； which favoured the longer rays in warm water； since they kept the organism afloat； while those with short rays sank and were eliminated。 If we put the question as to selection…value in this case； and ask how great the variations in the length of processes must be in order to possess selection…value； what can we answer except that these variations must have been minimal； and yet sufficient to prevent too rapid sinking and consequent elimination？ Yet this very case would give the ideal opportunity for a mathematical calculation of the minimal selection…value； although of course it is not feasible from lack of data to carry out the actual calculation。
But even in organisms of more than microscopic size there must frequently be minute； even microscopic differences which set going the process of selection； and regulate its progress to the highest possible perfection。
Many tropical trees possess thick； leathery leaves； as a protection against the force of the tropical rain drops。 The DIRECT influence of the rain cannot be the cause of this power of resistance； for the leaves； while they were still