HEREDITY, VARIATION AND DEATH 35 



of fission, while others reject conjugation as inadequate. The 

 reason of the adverse verdict has, I think, been an incomplete 

 half-hearted acceptance of the principle of evolution. Any 

 of the higher animals you like to choose is so wonderful in 

 its complexity, in the specialisation of its cells each organ, 

 brain, limbs, heart, blood - vessels, lungs and so forth, all 

 marvellous that to find the original fountain - head of all 

 this in anything that may happen in the lives of one-celled 

 animals seems to them ridiculous. But an evolutionist is bound 

 to seek for the causes of evolution in the most primitive organ- 

 isms known to us. And have not the most highly organised 

 animals sprung from the union of two simple cells ? Whatever 

 its source, there is reason to believe that the tendency to vary 

 is very strong and is only kept in check by wholesale elimina- 

 tion of individuals that deviate widely from the specific type. 

 Natural Selection maintains and strengthens heredity which, 

 apart from it, would speedily fail and lose its power to guide. 

 It is more difficult to account for the persistence of heredity 

 than for the lapses from it. 



It is possible to suggest another cause of variations, but it will 

 be found that I have already mentioned it by implication. There 

 is no life without motion, and motion certainly this is true of 

 the complicated manoeuvres that take place within the nucleus 

 during karyokinesis can hardly go on without change. Every 

 movement within a cell may cause some trifling rearrangement 

 among particles, some change in the relation of one particle to 

 another. The wonder is, not that alterations should take place, 

 but that the architecture should show such permanence and 

 constancy. It must be understood, then, that when I speak of 

 fission as a cause of variation, I include under the term the 

 movements within the nucleus that in many cases precede 

 fission. 



I believe that the infusorian's senility and death after multi- 

 tudes of fissions is due to excessive specialisation. Weismann 

 himself suggests that the specialisation of the somatic cells may 

 be the cause of death in the multicellular animals, the Metazoa, 

 but he rejects it for what seem to me inadequate reasons. In 



