IG-i REPRESENTATION OF PROTOMORPHIC ALTERNATION 



§ 7. Additional illustrations might be cited, but those 

 just given appear sufficient to establish a certain community 

 of nature in all cases of the kind, provided the survey be 

 extended over the whole course of phenomena intervening 

 between two successive returns of the act of digenesis. In 

 all cases it is probable that two successive steps can be dis- 

 tinguished, which may be termed respectively germinal and 

 embryonic — the later structure, which alone acquires the 

 typical characters of the species, arising from the earlier by 

 a more or less appreciable process of gemmation. In what 

 is called alternation of generations, this gemmation, in the 

 first place, is remarkably definite, the product frequently 

 becoming detached, and acquiring in some sort a distinct 

 individuality ; and, in the second place, it is generally 

 multiple, so as apparently to give rise to a new generation 

 of many separate zooids. We find, too, that the gemma- 

 tion is occasionally repeated many times over, so that a 

 whole race or series of o-enerations seems to orio-inate from 

 a single ovum. But however striking these points of dif- 

 ference may be, the transitional cases just cited indica.te 

 that the diversity is less in the essential nature of the func- 

 tion than in its being exaggerated, as it were, in the lower 

 species, so as to diverge from what may be considered, in 

 some sort, a process common to them and the higher — viz., 

 the gemmation, just referred to, of the embryonic structure 

 from the germinal mass. So long as the exaggeration is 

 merely in its distinctness (as in the Echinodermata) the af- 

 finity of the process to the normal course of embryogeny is 

 sufficiently apparent. Even when a new element of dis- 

 crepancy is introduced by multiple gemmation, we can still 

 find a parallel in the embryogeny of the higher species, 



own independent life, and may even, for ouglit that is known to the con- 

 trary, be capable of developing another Starfish, by a repetition of the 

 same process of gemmation, for evisceration, which is so readily bom by 

 the adult Holothuria, can hardly of itself be fatal to the Bipinnaria. 



