164 



THE ORIGIN AND EVOLUTION OF LIFE 



types which increasingly depend upon lateral motions of the 



body for propulsion and thus tend to lose the lateral fins and 



finally to assume (E) an 

 elongate, eel shape, en- 

 tirely finless, for pro- 

 gression along the bot- 

 tom; (C) the bottom- 

 living forms, in which 

 the body becomes later- 

 ally broadened, the head 

 very large relatively and 

 covered with protective 

 dermal armature, the 

 movements of the ani- 

 mals becoming slower 

 and slower as the dermal 

 defenses develop. This 

 law applies to all the 

 vertebrates, including 

 man, namely: the de- 

 velopment of armor is 

 pari passu with the loss 

 of speed. Conversely, 

 the gain of speed neces- 

 sitates the loss of ar- 

 mor. Smith Wood- 

 ward^ has traced similar 



radiations of body form in the historic evolution of each of the 



great groups of fishes. 



The interest of this fivefold law of body-form radiation is 



greatly enhanced when we find it repeated successively under 



' Smith Woodward, A., 1915. 



UPPERS 

 SILURIAN.' 



PALEOGEOGnAPHY, UPPER SILURIAN (SALINA) TIME 

 AFTER SCHUCHERT, , 

 NE DEPOSITS ^- CONTINENTAL DEPOSITS ^,V SALT DEPOSITS 



Fig. 45. 



North America in Upper Silurian 

 Time. 



During this period of depression of the Appala- 

 chian region and elevation of the western half of 

 the North American continent occurred the 

 maximum evolution of the most primitive armored 

 fishes, known as Ostracoderms, which were 

 widely distributed in Europe, America, and the 

 Antarctic. After Schuchert, 1916. 



