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THE GEEM-PLASM THEORY 389 



Crustaceans, Insects, and Birds, they attain to very different grades 

 of development even among the same species. Thus the birds of 

 Paradise are in most species brilhantly coloured and adorned with 

 decorative feathers only in the male sex, while the females are 

 simply blackish-grey, but there is a single species in which the males 

 are almost as soberly coloured as the females. Conversely, too, we 

 find that in parrots both sexes are usually coloured alike, but a few 

 species exhibit a totally different colouring in the two sexes. In the 

 same way the secondary sex differences may affect only a few parts 

 of the animal or many, while in a few species the sexes are so 

 divergent in structure that almost everything about them may be 

 called different. Examples of this are the dwarf males of most 

 Rotifers, and the males, more minute still in proportion to the females, 

 of the marine worm Bonellia viridis (p. 237). 



We have now to inquire what theoretical explanation of these 

 facts we can arrive at in accordance with the germ-plasm theory 

 That double determinants, male and female, for the differently formed 

 parts of the two sexes must be assumed to exist in the germ-plasm 

 has been already said, and we have to suppose that the same stinmlus 

 — usually unknown to us — which incites the determinants of the 

 primary sexual characters to activity also liberates those of the 

 secondary characters. But we may safely go a step further and 

 conclude that there are male and female ids, that is, that the nm le 

 and female determinants belong to different idsy'T!' infer this frouT 

 the fact that in some groups, such as the Rotifers and certain plant- 

 lice, the ova are sexually differentiated even at the time of their 

 origin. Males and females of these animals arise from different 

 kinds of eggs, which are even externally recognizable. Both develop 

 parthenogenetically, so that fertilization has nothing to do with it ; 

 from the first, therefore, they must contain ids which consist of deter- 

 minants of one sex alone. 



If this conclusion be correct, then the sexual equipment of the 

 determinants of the sexual characters must have taken place in 

 the course of phylogeny in such a way that each id was aftected 

 in one direction only, and we should thus have to assume male and 

 female ids, even before the separation of the sexes as males and 

 females, and the same conclusion must be extended to the primary 

 sexual characters. Only in this way can we understand the fact 

 that differences between the sexes, at first small, have increased in the 

 course of phylogeny to such complete divergence of structure as is 

 now exhibited in the forms we have named, Bonellia, the Rotifers, 

 and some parasitic worms. 



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