io8 INVERTEBRATE PALAEONTOLOGY 



traced seems to indicate that the principle applies 

 equally to large and small groups of organisms. 



Tendency to specialization appears to vary in inten- 

 sity and scope at different stages of -racial evolution. 

 There are three stages during which rapid morpho- 

 genetic acceleration typically occurs, coinciding fairly 

 closely with the youthful, mature and senescent phases 

 of race-life. Of these, the first is the most strongly 

 marked, and is usually productive of the most far- 

 reaching innovations. When the geological ranges of 

 the several classes of a phylum are compared, it is 

 commonly found that, while the periods of acme or 

 extinction may be very diverse, the times of inception 

 follow very shortly after differentiation of the whole 

 group. In the 'case of most Invertebrate phyla, this 

 quality is obscured by its pre-Cambrian remoteness, 

 but among Vertebrates it is very apparent. The rich 

 variety of the Cambrian fauna can doubtless be ascribed 

 to its operation. That the same tendency works through 

 smaller divisions is abundantly manifest. Indeed, the 

 rarity of cases where gradual transition from one family 

 to another can be demonstrated used to supply some 

 of the most powerful arguments of opponents of the 

 evolution-theory. However, analogy with individual life 

 would make such neanic acceleration postulated, were it 

 not actually observed. The early ontogenetic phases of 

 an organism are plastic and protean, as one structure 

 after another is produced and developed; relative stability 

 characterizes maturity. 



Among the many instances of rapidity of specializa- 

 tion in young groups, that shown by the Echinoidea is 

 well marked. After a series of experiments in Palaeozoic 

 times, the class acquired a certain fixity of character 

 in the Triassic Cidaridae. (The Perischoechinoidea, in 

 their manifold and extraordinary varieties, can perhaps 



