HEREDITY, VARIATION AND DEATH 31 



To the theory of the continuity of the germ-plasm Weismann 

 has linked another which is really independent of it his theory 

 of the evolution of death. I restate it for clearness' sake. 



" The unicellular organisms are immortal. It is true that they Weis- 

 die unless conjugation takes place. But conjugation is part of nj> ann ' s 

 their normal cyclic life. Death, owing to failure to conjugate, is t he evolu- 

 accidental, not natural. Conjugation is not in its origin a vitalis- V on , of 

 ing process. The part assigned to it is the production of varia- 

 tions. Those species in which it was the rule produced more 

 favourable variations than others in which parthenogenesis ob- 

 tained, and so they have prevailed in the struggle for existence. 

 Conjugation has thus come to be normal, and Natural Selection 

 has brought it about that all individuals 'which do not conjugate perish. 

 They are harmful to the species." 



To this account of the origin of death there is an obvious 

 objection. Natural Selection is only a regulating principle. It 

 can create nothing. Natural death exists now, therefore it 

 must have existed from the first. It may, however, have been 

 exceptional and abnormal before animal and vegetable forms 

 attained their present complexity, and through the working of 

 Natural Selection the exceptional may have become normal. 

 Let us take this to be Weismann's meaning. Even though 

 this is conceded, he is still open to the charge of invoking the 

 aid of Natural Selection as a deus ex machina whenever he is in 

 difficulties. "Worn out individuals," he says, "are not only 

 valueless to the species, but they are even harmful, for they 

 take the place of those which are sound. Hence, by the opera- 

 tion of Natural Selection, the life of our hypothetically immortal 

 individual would be shortened by the amount which was useless to 

 the species." x The individual in question is worn out by accidents, 

 the buffets and shocks of chance, not by disease, for where that 

 exists there must be natural death. In any case, Weismann says, 

 he is de trop, and Natural Selection is, therefore, ordered to 

 deprive him of immortality. But Natural Selection will not 

 carry out all and any orders that the scientist may issue, and 

 when she (to resort to personification for a moment) sets to 



1 Essays upon Heredity, vol. i. p. 14. 



