420 THE GERM-PLASM 



multicellular organisms, and the latter process is never entirely 

 wanting in any species, the germ-plasm of these forms consists 

 of many ids from different sources, — one half of them being 

 derived from the father, and one half from the mother, each 

 half, again, containing ids from the grandparents in varying 

 proportions ; the ids of the grandparents, moreover, are derived 

 from one, two, or even three great-grandparents, and so on. 

 As I have shown above, the proportions in which the individ- 

 ual ancestors may be represented by ids may vary very much ; 

 and consequently the germ-plasm of different individuals, even 

 when they are closely related, must always differ. 



Each id contains all the determining elements of the species, 

 but in a manner peculiar to the individual. If a determinant N 

 differs slightly in every id, it will also vary a little during growth 

 if exposed to modifying influences ; so that, for example, the 

 determinant N may remain unaltered in id A, while N^ varies 

 in B. On the other hand, the modifying influence of nutrition 

 may very likely be slightly different in A and in B, and may 

 produce a variation in N, while N^ remains unaltered. Sufficient 

 factors would thus be present to cause a variation of one or 

 more homologous determinants in certain, but not in all ids. In 

 the only carefully observed cases of blastogenic variation, due 

 to the direct influence of external conditions, — viz., those of the 

 climatic variations of Polyoiiunatus piilcBas^ — it is evident that 

 the influence of temperature was not quite uniform. Some of 

 the captive Neapolitan specimens of this species are darker 

 and others lighter, although they were raised in a room under 

 conditions as nearly the same as possible ; and in those 

 caught in the neighbourhood of Naples, the amount of black- 

 dusting — the character modified by the climate — varied con- 

 siderably. The same influences, even when they act during 

 many successive generations, do not necessarily cause the indi- 

 viduals of a species to vary to the same extent ; and I account for 

 this by the fact that the ids of every animal contain different 

 variants of a particular determinant N, some of which are more 

 susceptible to heat than others. The germ-plasm, as a whole, 

 will vary to a greater or less extent in proportion to the number 

 of the determinants which are more or less variable. 



Many enigmatical points, however, still remain. Thus the 

 transformation by heat of many of the original reddish-gold 

 wing-scales of P. phlceas into black ones does not occur evenly, 



