LAND LIFE BEGINS 37 



vances are caused and settled in the germ-plasm (the 

 seed). Do not believe it. The science of heredity- 

 is still very obscure and imperfect, so I am saying 

 little about the internal causes of those changes in 

 animals and plants which make evolution possible. 

 They are of the very first importance, but are still 

 obsciire. What is clear is that the changes in the 

 earth itself have had enormous influence in directing 

 these variations at birth into the formation of new 

 species. 



Here we have a striking illustration. The land 

 rises, the rivers flow more rapidly, and the need for 

 speed, which was already great on account of the 

 struggle of hunter and hunted, becomes greater than 

 ever. The fish family appears in the waters, begin- 

 ning with uncouth forms that no longer exist, passing 

 on to the shark, then branching rapidly into hundreds 

 of types. The fins were probably at first folds of the 

 skin, which were gradually strengthened by ribs of 

 cartilage. Bones were not yet developed. The lower 

 fishes of to-day (sharks, rays, etc.) have no bones. 

 They are the survivors of one of the earliest families. 

 The backbone is the chief new departure. The great 

 "back-boned" (vertebrate) family has begun. But 

 the backbone was at first not a bone at all. It was 

 just a stiffening rod of cartilage, and its evolution is 

 easily traced. 



We must turn now to another momentous con- 

 sequence of the geological changes which I have 



