352 THE GERM-PLASM 



CHAPTER XI 



DIMORPHISM AND POLYMORPHISM 



I. Normal Dimorphism 



The phenomena of reversion discussed in the last chapter 

 depend on the capacity possessed by organisms of conveying 

 in their idioplasm characters which they do not themselves 

 possess in the form of ' latent ^ primary constituents, and of 

 transmitting these to their descendants, in which they may, 

 under favourable circumstances, undergo development. 



It has hitherto been supposed that all the individuals of a 

 species possess these latent primary constituents in the same 

 degree ; and that, consequently, characters which are capable 

 of becoming occasionally manifest at all in any organic form 

 exist in a latent condition in all individuals of the species, their 

 subsequent appearance or non-appearance depending solely on 

 certain developmental conditions. Even Darwin was of this 

 opinion, as is shown by many passages in his works. He 

 imagined that latent ' gemmules ■ existed, for instance, of stripes 

 like those of the zebra in every horse, of the slate-blue colora- 

 tion of the rock-pigeon in every domesticated pigeon, and of 

 the two parental species in every hybrid. In the last chapter 

 I attempted to show that this may be true in many cases, — 

 such as, for instance, in the races of pigeons, — but that it is by 

 no means necessarily so ahvays, and that in numerous instances 

 certain latent ancestral characters are not present in all, but 

 only in a larger or smaller number of individuals of the form 

 in question. We have seen that the reducing division may, 

 indeed, from one generation to another, divide the germ-plasm 

 of the parent in such a manner that the germ-plasm of some of 

 the offspring receives no portions of the idioplasm of one of the 

 grandparents at all. The most striking example is seen in 

 plant-hybrids, in which reversion to one of the parental forms 

 may even occur amongst the oifspring of the hybrid. In spite 

 of the nearness of the generations between such descendants 



