200 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



3. INEXACT PARALLELISM OR C^ENOGENY. 



When the early or transitional stage of a higher 

 form is exactly the same as a permanent lower form, 

 the parallelism is said to be "exact." Such is the re- 

 lation of a Cnemidophorus gularis scalar is to a Cnemido- 

 ^phorus gularis gularis as to color characters ; and of an 

 Amblystoma tigrinum to a permanent breeding Siredon 

 lichenoides in characters of higher structural value. 



When the transitional stage of the higher only re- 

 sembles the lower form in some one or more features, 

 but not in all, the parallelism is said to be "inexact." 

 It is evident that "exact parallelism " can only exist 

 between ancestor and descendant in the same re- 

 stricted line, and can be therefore only demonstrated 

 in the case of the nearest relatives, between which a 

 perfect phylogeny is known. So soon as new subordi- 

 nate characters are assumed, or a change in the order 

 of appearance of characters supervenes, the parallel- 

 ism becomes "inexact," and such is the kind of paral- 

 lelism usually observed. And it is more inexact the 

 more widely removed in relationship are the forms 

 compared. Thus the parallelism between the embryo 

 man with five branchial slits, and the adult shark, is 

 very inexact ; but that between a true fish and a shark 

 is much less inexact. That between a higher and a 

 lower shark is still more exact, and so on. Exact par- 

 allelism in growth is called by Haeckel palingenesis 

 or palingeny. The growth which has, through changes 

 introduced subsequent to the origin of a line of de- 

 scent, become inexact, or "falsified," is termed by the 

 same author caenogenesis or caenogeny. 



