154 THE EVOLUTION THEORY 



polydactylisni of both right and left hands, and even of liands and 

 feet at once, do actually occur. Tliat the right and left hands, the fore- 

 and hind-limbs, are represented in the germ by particular deter- 

 minants, may be inferred from their frequently different phyletic 

 evolution into different forms of hand and foot, e.g. into flipper and 

 rudimentary hind-leg in the whale, as well as from the cases of 

 particulate inheritance, which are rare, but which undoubtedly do 

 occur, such as when, in Man, there is a maternal blue eye on one side 

 of the head and a paternal brown eye on the other. But almost more 

 striking than the differences betw^een these homologous or homotypic 

 |)arts are their points of resemblance, and these may probably be in part 

 referred to their disposition side by side and common history in the 

 germ-substance, although a far larger proportion of them are probably 

 due to their adaptation to similar functions, and are therefore to 

 be regarded as a phenomenon of convergence within the same 

 organism. 



We have already seen that the first increase in the growth of 

 one determinant means a withdrawal of nourishment, however slight, 

 from its neighbours ; this can, of course, be equalized again if the 

 claims on the common nutritive stream from another quarter are 

 at the same time diminished ; but it is possible that the claims from 

 another quarter may also be increased, and the withdrawal will then 

 be more marked, and the determinants being thus injured from two 

 directions at once will sink downwards with greater rapidity. But 

 it is also conceivable that the majority of determinants of a part may 

 vary upwards, and, by their combined increased power of assimilation, 

 direct towards themselves such a greatly increased stream of nourish- 

 ment that the whole organ — for instance, a particular feather in a bird 

 — varies in an upward direction, and becomes larger and larger, as we 

 see in the case of many decorative feathers ; or that certain deter- 

 minants vary only as far as some of their biophors are concerned, 

 and similarly for their determinates, as when a group of scales on 

 a butterfly's wing that had previously been black turn out a brilliant 

 blue. It can probably also happen that such variations within the 

 determinants are transmitted to neighbouring determinants because 

 the nutritive conditions which caused the first to vary have extended 

 to those about them. The increase of brightly coloured spots in birds 

 and butterflies gives us ground for concluding that there are processes 

 of this kind within the germ-plasm. 



I will refrain from following tliis idea into greater detail, and 

 translating the observaljle relations and variations of the fully-formed 

 parts of the body into the language of the germ-plasm ; but so much 



