DEATH IS A CONSEQUENCE OF EVOLUTION 149 



the ordeal of birth, growth, decline, and extinction. If it can 

 be said of species that they have no power of transcending a 

 certain limit of life, death must be for them, as for multicellular 

 organisms, a constitutional necessity. 



The death of multicellular organisms, after a certain age- 

 limit is reached, is a consequence of evolution itself. Unicellular 

 organisms do not succumb, as the multicellular organisms do, 

 to the natiixal wear and tear of life ; having reached a certain 

 stage of growth, the unicellular organism divides into two — each 



so completely disappeared that it is sometimes only discoverable as a 

 vestige in the embryo. All these transformations are adaptations to the 

 aquatic life, and have been called forth by the changed conditions of life 

 of the species. 



Delage, in his criticisms of the theory of selection {L'H^redite, etc., 

 pp. 844 ff.), has based his arguments on a misunderstanding. He says that, 

 in order that selection may favour the increase or decrease of any part of 

 the organism, it is essential that this increase or decrease should be advan- 

 tageous to the individual. This is incontestably true ; but Delage pro- 

 ceeds to ask, folio-wing Herbert Spencer, what advantage the whale has 

 gained from the progressive atrophy of its femur. The question seems to 

 have been answered by Weismann. We have seen in our study of ger- 

 minal selection that, although the continued existence of a part which has 

 been rendered useless owing to a change in the conditions of hfe may not 

 be in itself harmful to the species ; nevertheless, the determinants of this 

 useless part will continue to attract a stream of intragerminal nutrition 

 which would otherwise go to sustain the determinants of useful parts ; 

 and this must necessarily entail a weakening of the determinants of these 

 useful parts. Therefore, once a part has ceased to be of use, selection 

 necessarily intervenes by withdrawing the intragerminal nutriment from 

 the determinants of this useless part. We must also remember that 

 selection is not only exercised between individuals or between the organs 

 and parts of the same individual, but that it operates within the germ- 

 plasm itself. 



When Delage goes on to say that " une variation faible ne peut con- 

 stituer un grand avantage," he is making an assertion which it is im- 

 possible to prove, for we have no means of judging in many cases of the 

 biological value of a variation. It may often happen that a variation which 

 is apparently insignificant is, nevertheless, of biological value ; and, in 

 any case, even those minor variations which do not attain biological value 

 are none the less determined by germinal selection. Delage has only 

 considered natural selection. The most insignificant variation is, how- 

 ever, none the less due to selection within the germ-plasm. 



