ZOOGENESIS 



tions of those groups drawn up from and based entirely 

 on living types, and since none of these definitions of 

 the phyla or major groups of animals need be in any 

 way altered or expanded to include the fossils, it 

 naturally follows that throughout the fossil record 

 these major groups have remained essentially un- 

 changed. This means that the interrelationships be- 

 tween them likewise have remained unchanged. 



Strange as it may seem, the animals of the very 

 earliest fauna of which our knowledge is sufficient to 

 enable us to speak with confidence, the fauna of the 

 Cambrian period, were singularly similar to the ani- 

 mals of the present day. In the Cambrian crustaceans 

 were crustaceans, echinoderms were echinoderms, 

 arrow-worms (fig. 6x, p. iii) were arrow-worms, and 

 mollusks (figs. 45-52., p. 97) were mollusks just as 

 unmistakably as they are now. 



In order to illustrate the diversity of life in the 

 Cambrian seas and to bring out its striking general 

 similarity to the life of the present time, let us briefly 

 note the types of animals which flourished at that 

 far distant period. 



In Cambrian times crustaceans were represented by 

 phyllopods, trilobites (fig. 3x, p. 55) and merostomes 

 (fig. 31, p. 55); among the echinoderms there were 

 crinoids (fig. 6, p. 5), cystideans (figs. 37, 38, p. 55), 

 and elasipod and other holothurians; chastognaths 

 (fig. 62., p. Ill) or arrow-worms, lamp-shells (fig. 60, 

 p. Ill) or brachiopods, and graptolites (figs. 39, 40, 

 p. 55) were present; of the annelids or jointed worms 

 we know polynoids, nereids (fig. 85, p. 161), gephy- 



