XIV 
FUNDAMENTAL PROBLEMS 
439 
of the specific germ-plasm, which the parent egg-cell contains, 
is not used up in producing the offspring, 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, whence arises a 
minglingof 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 grandparents, 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 cause 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. 1 
1 There are many indications that this explanation of the 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 
