418 Causes and Course of Organic Evolution 



the fresh-water or terrestrial invertebrate half of animal life 

 had already reached its highest platform, and that the verte- 

 brate half was already well under way. So between the mid- 

 archsean and the late cambrian we unquestionably have to 

 look for the beginnings and the full development of the inver- 

 tebrate line, as well as for the established foundation of verte- 

 brate organisms; while from the close of the cambrian till 

 our day both invertebrate and vertebrate groups have been 

 some of them evolving, some stagnating, some devolving, many, 

 ^'ery many, disappearing, in the great struggle foi existence. 



Second: During the evolution of the invertebrates into the 

 vertebrates the writer would suggest that two main lines have 

 arisen, which show their first divergences from a common 

 stock in the group Rotifera above considered. These are 

 the soft-bodied or scaleless, and the firm-bodied or scaled, 

 protovertebrates, that are first shadowed forth in members of 

 the above group. 



Third: In comparisons of the Vertebrata hitherto made, 

 it has not generally been recognized that two such lines can 

 be traced, one that includes the Cyclostomata, Cseciliada, 

 Urodela, Marsupialia, and higher mammals; the other that 

 includes the scaled or plated fishes, the ancient or scaled and 

 plated amj>hibians, the reptiles, and the feathered birds. 



Fourth: That in spite of their soft bodies and therefore 

 perishable characters, sufficient living and fossil evidence can 

 be secured that enables us to trace the main line of ascent 

 from the rotifers through turbellarians, nemerteans, cyclo- 

 stomes, and urodeles to higher mammals, and so that Huxley's 

 contention is correct when he connected the Amphibia (Ba- 

 trachia) and Mammalia in a continuous ascending series. 



With the above principles to guide us we may first review 

 general features of resemblance from rotifers to the mammals, 

 and thereafter, in the remainder of this chapter and in the 

 next, treat with minute detail the structural characters of 

 the Nemertinea, the Cyclostomata, and higher groups. 



The body surface in the Rotifera, in rhabdocoel Turbel- 

 laria, in Nemertinea, in the ammocoete cyclostome larva, in 



