54 BIOLOGICAL SOCIETY OF WASHINGTON. 



becoming larger and longer, teeth shorter and more generalized, 

 skeletons less characteristic; rhinoceroses with cutting teeth, and 

 more slender forms ; tapir-like animals without the peculiar tapirine 

 teeth, with rhinocerotoid skulls, and with otherwise modified 

 structure ; all these accompanied by innumerable other modifica- 

 tions, till finally we are almost at a loss to tell whether it is a horse- 

 like, a rhinocerotoid or a tapiroid animal that is before us, and they 

 become lost in earlier forms with special characters of their own. 

 And as we go still further back we are confronted with still other 

 forms that are connected by series projected backward from the 

 ruminants and from the elephantids. We do, in fine, know the 

 genealogy of our own contemporaries — imperfectly it is true, but 

 still we know it. 



It was objected that animals were segregated by such very wide 

 intervals that they must be isolated in different branches, and that 

 there could be no community of structure between such branches ; 

 they expressed fundamentally different plans of structure. 



One by one zoology, anatomy, and embryology supplied the links 

 between the old branches ; the branches were at length completely 

 uprooted, and it has even become a matter of simple convention 

 what should be considered major groups. Plans of structure can no 

 longer be claimed to be peculiar to different types. 



That branch of which man is the primate — the vertebrates — was 

 supposed to be perfectly unassailable and isolated ; but zoology and 

 anatomy have revealed to us amphioxus, and embryology the earlier 

 stages of the tunicates. The evidence is now conclusive that these 

 forms which once appeared to be among the most distant are now 

 the most closely related. The affinities of the tunicates with inver- 

 tebrates are evident, and thus we may look far back to that time 

 when vertebrates did not exist, but when the common ancestors, from 

 which they and the related invertebrates should diverge, held sway. 



It was even pretended that the evidence was insufficient to show 

 that variation was possible or could be propagated. 



From every hand testimony was forthcoming. The breeder could 



