388 THE EVOLUTION THEORY 



This ease may be compared in some respects with the permanent 

 colour-adaptation of those caterpillars, in regard to which Poulton 

 demonstrated that they become almost black if they are reared on 

 blackish- brown bark, light brown on light bark, and green if they 

 are kept among leaves, and in all cases permanently so. In this case 

 also the implicated pigment-cells of the skin may develop in three 

 ways, according to whether this or that quality of the light releases 

 this or that determinant. 



But in many cases we do not know the quality of the liberating 

 stimulus, and must content ourselves with imagining it. This is so 

 in the case of dimorphism of the sexes. It is clear that in the males 

 of a species the germ-cells develop quite otherwise than they do in 

 the females, that different determining elements attain to activity in 

 each sex, and since the primary constituents of both sexes must be 

 contained in most animals in the ovum and in the spermatozoon, 

 we must assume that in both there are at once 'ovogenic' and 

 ' spermogenic ' determinants, of which, however, only one kind becomes 

 active in a given individual. There are, however, both among plants 

 and animals hermaphrodite individuals, in which both kinds of sexual 

 products are developed simultaneously or successively. 



It is not only the primary sexual characters, however, that 

 compel us to the assumption of double determinants in the germ- 

 plasm, the secondary sexual characters do so too. We know very 

 well in relation to ourselves that ' the beautiful soprano voice of the 

 mother may be transmitted through the son to the grand-daughter, 

 and that the black beard of the father may pass through the daughter 

 to the grandson.' Thus both kinds of sexual characters must he 

 present in every sexually differentiated being, some visible, others 

 latent. In animals the determinants are sometimes handed on from 

 germ-plasm to germ-plasm through several generations in a latent 

 state, and only make their appearance again in a subsequent 

 generation. This is the case in the water-fleas (Daphnids) and the 

 plant-lice (Aphides), in which several exclusively female generations 

 succeed one another, and only in the last of them do males occur 

 again side by side with the females. 



. The germ-plasm of the ovum which is ripe for development must 

 thus contain not only the determinants of the specific ova and sperms 

 of the species, but also those of all the male and female sexual 

 characters, which we discussed at length in the section on sexual 

 selection. I then showed that these secondary sexual characters 

 differ greatly in range and in strength, that among lower animals 

 they are almost entirely absent, and that among higher forms, such 



