ALTERNATION OF GENERATIONS. 133 



not usually furnish instances of pro-embryonic forms. We 

 may observe indications, indeed, of such a tendency even in 

 the Cestoid order, which has been referred to as furnishin"- 

 the most satisfactory examples of the association, for we 

 lind that when gemmation is very marked in one stage, 

 whether protomorphic, as in Echinococcas, or gamomorphic, 

 as in the common Tapeworm, it is in general proportionally 

 in abeyance in the other. 



§ 6. There are, however, I conceive, certain cases of 

 alternation, which are not properly referable either to the 

 protomorphic or gamomorphic stages of the genetic cycle, 

 but depend rather on the occurrence of gemmation in the 

 intermediate or orthomor]^)hic stage — that is, during the sub- 

 sistence of a more fully developed condition of the typical 

 organization, but prior to the maturation of the sexual 

 org-ans. This I believe to be the case with the alternation 

 which is so striking a feature in the reproduction of the 

 Ajj/iides. The remarkable phenomena attending the propa- 

 gation of these insects is too well known to require any 

 detailed account to be given here, and I shall conhne myself 

 therefore to such points as have some bearing on its relation 

 to the other forms of alternation, especially as farther 

 reference will have to be made to the case, in connection 

 with the enquiry into the difference between ova and gemmie. 



The eggs of the Aphis are laid in autumn, and hatched, 

 as usual, in spring, but the primary larvas nevei^'attain the 

 full completeness of the insect type. They resemble the 

 woridiig bees in neutrality of sex, though certainly not in 

 sterility, as by a process of internal gemmation they 

 produce large broods of young, resembling themselves both 

 in their low grade of development and in their unassisted 

 powers of increase.* But as the larvse are incapable of 



* As this process of multiplication may go on the whole summer for 

 nine or ten generations, an easy calculation will show that many millions 

 of Aphides may spring from a single larva. 



