ALTERNATION OF GENERATIONS. 135 



epoch in the life history ; we can readily distinguish a 

 point of time when the cellular germ-mass first presents 

 the incipient traces of that structure which is characteristic 

 of the Vertebrate, Articulate, or other leading type of or- 

 ganization ; but we cannot draw such a definite line of 

 demarcation between any two successive periods of the 

 ensuing embryonic development : the subsequent changes 

 are of degree rather than of kind a gradual, and, on the 

 whole, a continuous unfolding of the perfection of the type 

 from the rudimentary traces which constitute its first com- 

 mencement. Undoubtedly, we meet occasionally with 

 arrests of development more or less complete and extended 

 and nowhere more markedly than in the class of insects 

 but these are all of an irregular or adventitious nature, 

 as is shown by the entire absence, in many cases, of any 

 break in the continuity of the process, and by their varia- 

 bility when they do occur. If, then, we regard embryogeny, 

 or larval development, as constituting the initial state of the 

 typical or orthomorphic stage but as, on that very account 

 strictly belonging to it then, we must refer the alternation 

 of the Aphides to the commencement of this stage rather 

 than to the protomorphic, because the organization has 

 already acquired that partially advanced development cha- 

 racteristic of the larvse of other insects, before the process 

 of gemmation comes into play. We cannot say here that 

 the primary product of impregnation buds off a set of em- 

 bryos of a higher organization ; it is rather a larva that 

 is, a naked embryo already so far advanced on the insect 

 type, that buds off a series of similar larvae, the last only 

 of which become perfect insects.* 



* Dr. Carpenter, while he virtually admits the distinction of the forms 

 of alternation here termed Protomorphic and Gamomorphic, by compar- 

 ing the zooids of the latter to detached reproductive organs, and by term- 

 ing the former a process of " larval gemmation," at the same time uses 

 this expression to include the development of the precursory forms both 



