REPRODUCTION. 675 



Under these circumstances we see that Brooks' theory 

 is untenable, and that the male cell is not to be regarded 

 as simply stimulating the female cell to development, but 

 as contributing materially to the development of the embryo. 

 In continuing to use the terms "fertilisation" and "conju- 

 gation" with reference to the sexual process, we must bear 

 in mind that they have only a morphological significance; by 

 conjugation we mean the coalescence of externally similar 

 gametes; by fertilisation we mean the coalescence of ex- 

 ternally dissimilar gametes: the physiology of the process 

 is the same whether the coalescing gametes are externally 

 similar or dissimilar. 



In conclusion, we have to discuss the theories of variation 

 to which allusion was made above; but in doing so nothing 

 more than a very general treatment of the subject will be 

 attempted. We will begin by setting forth, somewhat more 

 fully than we have done as yet, Darwin's views, and then 

 we will briefly discuss those views which differ from his. 



In all cases, says Darwin, there are two factors in vari- 

 ation; the nature of the organism, which is the more im- 

 portant, and the nature of the conditions of life. By the 

 "nature of the organism" is meant especially the variability 

 of the organism, for, clearly, if an organism is not variable 

 it cannot vary. The first point which we have to deal with is 

 then the origin of variability. Some authors regard varia- 

 bility as an ultimate fact, and as much an aboriginal law 

 as inheritance. Darwin, however, does not accept this view, 

 but comes to the conclusion that variability of every kind is 

 directly or indirectly caused by changed conditions of life. 

 "If," he goes on, "it were possible to expose all the in- 

 dividuals of a species during many generations to absolutely 

 uniform conditions of life, there would be no variability." A 

 change in the conditions of life has the effect, which Darwin 

 terms the indefinite effect, of promoting the plasticity of the 

 organism, thus leading to much fluctuating or indeterminate 

 variation. In support of Darwin's view it may be again 

 pointed out that cultivated plants are far more variable than 

 wild ones, and this may be correlated with the fact that 



432 



