348 THE EVOLUTION THEORY 



played any part in the matter, it would be quite unintelligible why 

 the protective colouring should occur only where it acts as a 

 protection, and why, for instance, it should not appear sometimes 

 upon the upper surface of the butterfly wing, or upon the posterior 

 wings which are covered when the butterfly is at rest. We have 

 already studied in detail the precision with which the coloration is 

 localized on minute points and corners of the wing: this can only 

 be understood if natural selection works with the certainty of 

 a perfect mechanism. Chance only comes into the matter in so far 

 as it depends upon chance whether the relevant determinants in 

 one id or another are to vary in the direction of plus or minus ; but as 

 the germ-plasm contains many ids, and chance may decide it difierently 

 in each of these, the presence of a majority of determinants varying 

 in a desirable direction does not depend upon chance, for if they are 

 not contained in one individual they are in another. It is only 

 necessary that they should be present in some, and that these should 

 be selected for reproduction. 



We must therefore regard natural selection, that is to say, 

 personal selection, as a mechanical process of development, which 

 begins with the same certainty and works ' in a straight line ' towards 

 its ' goal,' just as any principle of development might be supposed to 

 do. Fundamentally it is after all a purely internal force which gives 

 rise to evolution, the power of the most minute vital units to vary 

 under changing influences, and it is only the guidance of evolution 

 along particular paths that is essentially left to personal selection, 

 which brings together what is useful and thus determines the 

 direction of further evolution. If we bear in mind that even the 

 minutest variations of the biophors and determinants express nothing- 

 more or less than reactions to changed external conditions in the 

 direction of adaptation, and that the same is true of each of the 

 higher categories of vital units, whether they be called cell, tissue, 

 organ, person, or conn, we see that the whole evolution of the forms of 

 life upon the earth depends upon adaptations following each other in 

 unbroken succession, and fitting into each other in the most complex 

 way. The whole evolution is made possible by the power of varia- 

 tion of the living units of every grade, and called forth and directed 

 by the ceaseless changes of the external influences. I said years ago 

 that everything in organic evolution depended upon selection, for every 

 lasting change in a vital unit means adaj)tation to changed external 

 influences, and implies a preference in favour of the parts of the 

 unit concerned, which are thereby more fitly disposed. 



In this sense we can also say that the species is a complex of 



/ 



