DERIVATION OF ORGANIC BEINGS. 15 



animal life. Where the gemmae do become detached, how- 

 ever, the case may assume the aspect of a form of alternation, 

 one of the most striking examples — that occun-ing in the 

 propagation of the Aphides — being, as I believe, referable to 

 this head. 



In cases, therefore, where alternation is an obvious and 

 well recognized occurrence, I would distinguish these three 

 varieties : — 



1. That in which the gemmation occurs in the proto- 

 morphic or germinal stage, prior to the appearance of the 

 typical organization ; 



2. That in which it occurs in the gamomorphic or later 

 stage of the life history — that is, the connection with the 

 maturation of the reproductive organs ; and 



3. That in which it occurs in the orthomorphic or inter- 

 mediate stage — -that is, during the manifestation of a more 

 fully developed condition of the typical organization, but 

 prior to the maturation of the sexual organs. 



§ 6. But, even in the higher forms of life, where we no longer 

 find any obvious alternation, I believe that phenomena occur 

 which are in some degree representative, and which admit 

 of the same sort of classification. Thus, though the well- 

 marked cases of alternation, due to the evolution of proto- 

 morphic zooids, are confined to a few of the lower orders, a 

 certain nisus, or tendency in this direction, — a fresh start, 

 as it were, in the course of germinal development — may be 

 traced with more or less distinctness in all cases of 

 embryogeny, as in all instances there is formed first a 

 cellular germ-mass, from one point of which there is subse- 

 quently developed a new axis of embryonic gro^Hh. And 

 as the appearance of the new centre of organization in the 

 early germ may stand as representative of protomorphic 

 alternation, so to the contrasted form marked by the produc- 

 tion of sexual or gamomorphic zooids, we may trace a certain 

 correspondence in the maturation of the reproductive organs. 



