178 THE GERM-PLASM 



pools and lakes which never become dried up, produce summer- 

 eggs alone for a large number of generations, and only give 

 rise to eggs of the other kind on the approach of winter ; and 

 these, on the death of the animals which produced them, ensure 

 the continuance of the species in the following spring. 



The occurrence of changes in ihtjinal stages of ontogeny must 

 be accounted for in a similar way. 



In plant-lice belonging to the genus Aphis, the fertilised tgg 

 gives rise to females, which are, however, incapable of being 

 fertilised, for the receptaculum seminis is wanting, and this is 

 essential in the process. Their eggs are, however, capable of 

 undergoing development in the ovary parthenogenetically. The 

 resulting offspring give rise to similar females possessing no 

 arrangements for fertilisation, and these again produce others of 

 the same kind. Ultimately, however, one of these gives rise 

 parthenogenetically to females which are capable of fertilisation, 

 as well as to males. The two sexes as a rule differ as regards 

 the shape and colour of the body, apart from the structure of the 

 reproductive organs and sexual products, but the embryogeny of 

 these sexual animals is similar to that of the others. 



In this case, therefore, the determinants of the mature animal 

 become modified in the parthenogenetic generations, for sexual 

 reproduction is the more primitive of tlie two forms of the proc- 

 ess. If therefore we make the assumption, which, however, 

 is not a strictly correct one, that the sexual generations have 

 remained quite unaltered since the introduction of alternation of 

 generations in these animals, we should have to represent the 

 phyletic change in the germ-plasm as being of such a nature as 

 to cause the degeneration of the determinants of the seminal 

 vesicle in one half of the ids, and to produce a change in other 

 determinants, such as those which control the colour of the in- 

 tegument, for instance. The modified, as well as the unmodified 

 ids, must be contained within the same germ-plasm, but they 

 control the egg alternately, and never become active at the same 

 time. 



In this instance the generations which have been interpolated 

 have only suifered a slight change as regards the structure of the 

 whole body. But in many cases of alternation of generations 

 very important differences of structure occur, so that not infre- 

 quently one might easily believe that the different generations 

 belong to two entirely different groups of animals. 



