148 DEVELOPMENT AND HEREDITY. 



On the other hand, animals whose environment 

 has changed little or not at all in the course of 

 geologic time, have changed very little. Thus take 

 the marine fauna, for example : the sharks of to-day 

 differ very little from the oldest fossil fishes, and the 

 echinoderms seem, in some cases, scarcely to have 

 altered since archsean times. The echinoderms are 

 so well defended by their armour that they have but 

 few enemies. Their greatest struggle is with their 

 own kind ; and this is not battle, but simply crowd- 

 ing each other out of existence. They have nearly 

 the same conditions on the sea-bottoms of to-day 

 that existed there in past geologic ages. The same 

 is true for the sharks. They were lords of the ear- 

 liest oceans, and have remained so until now. In 

 that earliest time when all fishes were nearly alike 

 in structure, great size would have been an advan- 

 tage, and it is apparently by their size that the 

 sharks have held their own. Fishes that were un- 

 able to cope with them must have been driven to 

 the estuaries and rivers, where, among the rocks and 

 debris of flooded currents, we may suppose they de- 

 veloped their ganoid armour. A few ganoids of great 

 size still inhabit the fresh water, whence their de- 

 scendants must have returned, as the osseous fishes 

 to the oceans. Farther on in the course of evolu- 

 tion, we must suppose that forms which could not 



