XIV FUXDAMEXTAL TROBLEMS 439 



of the specific germ-plasm, which the j^arent egg-cell contains, 

 is not used up in producing the offspring, but is reserved un- 

 changed to produce the germ-cells of the follo^\'ing generation. 

 Thus the germ-cells — so far as regards their essential part the 

 germ-plasm — are not a product of the body itself, but are 

 related to one another in the same way as are a series of 

 generations of unicellular organisms derived from one another 

 by a continuous course of simple di^^sion. Thus the question 

 of heredity is reduced to one of growth. A minute portion 

 of the very same germ-plasm from which, first the germ-cell, 

 and then the whole organism of the parent, were developed, 

 becomes the starting-point of the growth of the child. 



The Cause of J\niation. 



But if this were all, the offspring would reproduce the 

 parent exactly, in every detail of form and structure ; and 

 here we see the importance of sex, for each new germ grows 

 out of the united germ-plasms of two parents, whence arises a 

 mingling of their characters in the offspring. This occurs in each 

 generation ; hence every indi^-idual is a complex result repro- 

 ducing in ever-varying degTees the diverse characteristics of his 

 two parents, four grandparents, eight great-grandparents, and 

 other more remote ancestors ; and that ever-present individual 

 variation arises which furnishes the material for natui^al selec- 

 tion to act upon. Diversity of sex becomes, therefore, of primary 

 importance as tlie cause of variation. Where asexual genera- 

 tion prevails, the characteristics of the indi^rdual alone are 

 reproduced, and there are thus no means of effecting the 

 change of form or structure required by changed conditions of 

 existence. Under such changed conditions a complex organ- 

 ism, if only asexually propagated, would become extinct. But 

 when a complex organism is sexually propagated, there is an 

 ever-present cause of change which, though slight in any one 

 generation, is cumulative, and under the influence of selection 

 is sufficient to keep up the harmony between the organism 

 and its slowly changing en\Tronment.^ 



^ There are many indications that this explanation of the canse of variation 

 is the true one. Mr. E. B. Poulton suggests one, in the fact that ]jartheno- 

 genetic reproduction only occurs in isolated species, not in gTonp.s of related 

 species ; as this shows that parthenogenesis cannot lead to the evolution of 



