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 occurring 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 growth. 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. 



