XIV FUNDAMENTAL PROBLEMS 439 



of the specific germ-plasm, which the parent egg-cell contains, 

 is not used up in producing the off'spring, but is reserved un- 

 changed to produce the germ-cells of the following 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 division. 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 Variation. 



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, Avhence arises a 

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

 generation ; hence every individual is a complex result repro- 

 ducing in ever- varying degrees the diverse characteristics of his 

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

 other more remote ancestors ; and that ever-present individual 

 variation arises which furnishes the material for natural selec- 

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

 importance as the cmcse of variation. Where asexual genera- 

 tion prevails, the characteristics of the individual 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 environment.^ 



' There are many indications tliat this explanation of tlie cause of variation 

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

 genetic reproduction only occurs in isolated species, not in groups of related^ 

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



