THE GERM-PLASM THEORY 



Crustaceans, Insects, and Birds, they attain fco vrery differei 

 of development even among the same Bpecies. Thus the bii 

 Paradise are in most species brilliantly coloured and adorned with 

 decorative feathers only in the male sex, while the females 

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

 are almost as soberly coloured as the females. Conversely, b 

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

 species exhibit a totally different colouring in the two In the 



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

 of the animal or many, while in a few species tip- 

 divergent in structure that almost everything about them may be 

 called different. Examples of this are the dwarf males "i' n 

 Rotifers, and the males, more minute still in proportion to the femal< 

 of the marine worm Bonellia viridis (p. 227). 



We have now to inquire what theoretics 1 explanation of th< 

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

 That double determinants, male and female, for tin- differently formed 

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

 has been already said, and we have to suppose that tin- same stimulus 

 — usually unknown to us — which incites the determinants <•!' tip- 

 primary sexual characters to activity also liberates til"-,, 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 male 

 and female determinants belong to different ids. I infer this from 

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

 lice, the ova are sexually differentiated even at the time "I' 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 fco 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 

 determinants of the sexual characters must haw taken place 

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

 in one direction only, and we should thus ha v.- t<> assume male 

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

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

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

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



course of phylogeny to such complete diver-.-,,. I structu 



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



and some parasitic worms. 



r, b 



1. 



