Evolution of Animals 413 



exhibit so many features of diversity in themselves, that the 

 HkeHhood of land forms having been evolved of equal or even 

 of greater complexity than the marine types is extremely likely, 

 specially if we can connect these with higher fresh-water illus- 

 trations. In other words, we do not hesitate to attempt the 

 construction of a composite morphological photograph from 

 the species of any order, if such dovetails into and elucidates 

 a gioup of species from an order below or above. 



In starting on such a task with man, the anthropoid apes, 

 the lemurs, the lower mammals, and some still more piimitive 

 forms, as increasingly advancing types from the last- to the 

 first-named, what we might expect to deal with throughout 

 the entire scale of progress would be graded examples, that 

 showed in common a soft body of rounded or somewhat dorsi- 

 ventrally flattened outline and ^dth bilateral symmetry; that 

 had a skin devoid of scales, scutes, or feathers, but which was 

 rich in mucous or sebaceous skin glands, and bore cilia bristles 

 or hairs; that showed a somewhat enlarged head with dorsally 

 placed brain which connected with olfactory, optic, and audi- 

 tory sense-centers; that had some active, energized, and adapt- 

 able structure or structures that would bring it into constant 

 and varied contact with its environment; that possessed a 

 dorsal nerve cord and lateral nerves connected by transverse 

 threads; that exhibited no distinct "somites," but had an 

 internal, more or less segmental, arrangement of the myotomes, 

 the nerves, the blood vessels, the excretory and the reproductive 

 organs; that had a more or less discoid sucking mouth and 

 simple alimentary canal; that showed a dorsal notochord or 

 evident stages originating it and explaining its ontogeny; in 

 which the egg segmented holoblastically, and from which an 

 embryo was derived, that was wholly or in part ciliate. 



Since practically all metazoan organisms start with an 

 egg, the last i)hase in the above group of requirements re- 

 presents the one that we may consider first, as it was doubtless 

 the one that originated the metazoan group. 



In the holotrichous ciliate Infusoria there are still ])reserved 

 to us the i)rimitive unicellular nucleate organisms from which 



