THE ARGUMENTS FROM EMBRYOLOGY. 377 



and will show its effects in all kinds of ways and degrees, 

 according to the ancestral history of each species. Suppos- 

 ing it were possible for a race of organisms to have con- 

 tinued propagating itself through an indefinitely-long 

 period without any change of conditions, necessitating 

 change of structure ; there would be reached so complete a 

 congruity between the organic aggregate and its physiologi- 

 cal units, that the units would arrange themselves directly 

 into a structure like that of the adult organism : the germ 

 would put on the proper characters of the species, with little 

 or no transposition of substance. But in the absence of 

 any such constancy of conditions and structure, w^hat may we 

 expect ? We may expect that where the conditions and 

 structure have been most constant, the mode of develop- 

 ment will be the most direct ; and that it will be the most 

 indirect, where there have been the greatest and most 

 numerous changes in the habits and structures of ancestral 

 races of organisms. And we may also expect that develop- 

 mental changes corresponding to early ancestral forms, will 

 undergo an obliteration that is great in proportion to the 

 fixity of organization that has been since maintained. The 

 facts appear in harmony with this conclusion. We see a 

 comparatively- direct development in those inferior types of 

 animals, which show us, by their inferiority, that they have 

 not, since the commencement of organic life, passed through, 

 many sets of changes. And where we find direct de- 

 velopment among higher types of animals, it characterizes 

 the simpler rather than the more complex members of the 

 types. 



Between different parts in the same embryo, there are un- 

 likenesses in the method of formation, which seem to have 

 kindred meanings. The heart, of which the development is 

 in great measure direct, is an organ that appears compara- 

 tively early among the ascending grades of organic forms ; 

 and having appeared, retains throughout the character of a 

 hollow muscle. Conversely, the organs which develop with 



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