Importance of small differences 25 



are augmented in the course of innumerable generations, because 

 their possessors more frequently survive in the struggle for existence. 



($) 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 in- 

 significant 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 be- 

 ginnings 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 thick- 

 ness 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 Radiolariaus 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 signifi- 

 cance (Hacker). The same thing has been shown by Schiitt 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 



