HEREDITY, VARIATION AND DEATH 21 



to modifications in the arrangement of characters, the other to a 

 reduction in the amount of the plasm, similar phenomena ought 

 to be observable in the maturation of the male or sperm-cell. 

 This has been shown to be true of the sperm-cells of ascaris 

 (a nematode worm) ; there are two cell-divisions : as in the egg, 

 the amount of the chromatin is reduced by one-half, and here 

 too the germ-plasm is non-sexual : the sexual characteristics of 

 the cell are secondary. 



" Each of the two cells that are to fuse is made up of very 

 minute vital particles called biophors ; these combine together 

 to form determinants, each representing a bodily characteristic, 

 and these determinants are grouped into ids. The cell not only 

 contains thousands of determinants or characters each part of 

 the body that has a character of its own must have its own 

 determinant but it must have, besides that, a definite archi- 

 tecture. Each of the combining cells has a very similar, but not 

 the same, architecture, and the object of the fusing of the two is 

 to produce variations without which we should not be able to 

 account for the evolution of different species. It is certainly not 

 a vitalising process, producing rejuvenescence, for it has been 

 shown that in the unicellular organisms, the corresponding pro- 

 cess, conjugation, very much delays reproduction by fission. 

 Parthenogenesis proves that an egg, without fertilisation, is 

 capable of reproduction. 



" But Natural Selection could not go on without variations, 

 and in conjugation and sexual union is the chief source from 

 which variations arise. In unicellular organisms, it is true, 

 variations are largely due to external conditions. But this 

 cause is of quite minor importance among the metazoa, for in 

 them the reproductive cells are isolated, and it is important to 

 remember that the environment can only affect the germ-cells 

 by acting directly upon them. In parthenogenetic eggs, if we 

 neglect the insignificant influence of external conditions, the 

 preliminary division of the nucleus is the only source of varia- 

 tions, and it is found that animals born from such eggs do vary. 

 But far less diversity is likely to result in this way. Hence 

 conjugation and sexual union have arisen ; they supply to 



