v.] IN THE THEORY OF NATURAL SELECTION. 341 



a relatively rapid growth of the idioplasm derived from one of 

 the parents would ensue, and with it the preponderance of the 

 hereditary tendencies of the parent in question. Now, it is 

 obvious that no two cells of the same kind are entirely identical, 

 and hence there must be differences in their powers of assimi- 

 lation. Thus the varying hereditary powers of the egg-cells 

 produced from the same ovary become explicable, and still 

 more easily the varying powers of the germ-cells produced in 

 the ovaries or testes of different individuals of the same 

 species ; most easily of all the differences observable in this 

 respect between the germ-cells of different species. 



Of course, this hereditary power is always relative, as may 

 be easily proved by cross-breeding between different species 

 and races. Thus when a fantail pigeon is crossed with a 

 laugher, the characters of the former preponderate, but when 

 crossed with a pouter the characters of the latter preponderate '. 

 The facts afforded by cross-breeding between hybrids and one 

 of the pure parent species, together with a consideration of the 

 resulting degree of variability, seem to me to be even more 

 unfavourable to Brooks' view. They appear to me to admit of 

 an interpretation different from that brought forward by him ; 

 and when he proceeds to make use of secondary sexual 

 characters for the purpose of his theory, I believe that his 

 interpretation of the facts can be easily controverted. It is 

 hardly possible to conclude that variability is due to the male 

 parent, because the males in many species of animals are more 

 variable, or deviate further from the original type, than the 

 females. It is certainly true that in many species the male sex 

 has taken the lead in processes of transformation, while the 

 female sex has followed, but there is no difficulty in finding a 

 better explanation of the fact than that afforded by the assump- 

 tion 'that something within the animal compels the male to 

 lead and the female to follow in the evolution of new breeds.' 

 Brooks has with great ingenuity brought forward certain in- 

 stances which cannot be explained with perfect confidence by 

 Darwin's theory of sexual selection, but this hardly justifies 

 us in considering the theory to be generally insufficient, and 

 in having recourse to a theory of heredity which is as com- 



^ Darwin, 'Variation of Animals and Plants under Domestication,' 

 1875, Vol. II. p. 41. 



