ORIGIN OF VARIATIONS. 149 



maintain themselves against the competition in the 

 water were driven to living an amphibious life partly 

 on land, and so gave rise to reptiles and mammals. 

 A similar course of evolution seems to have occurred 

 from the worms through the crustaceans to the 

 insects. We see, therefore, how important has been 

 the effect of change of environment. It is sugges- 

 tive also to observe that, in primitive types, devel- 

 opment of size seems to precede development of 

 diversity in structure, — thus, the sharks in salt 

 water, the ganoids in fresh water, the carboniferous 

 amphibians and reptiles, and, finally, the modern 

 whale and elephant, as primitive forms among marine 

 mammals and herbivores. 



In order to clearly illustrate this idea of the 

 nature of variations of growth and development, 

 let us suppose that a species of carnivorous animals 

 should gradually accustom themselves to live more 

 and more around and in water, in order to eke 

 out the naturally insufficient food supply, by feed- 

 ing on fishes, and also to render themselves securer 

 from larger enemies. Where the life of the animal 

 had been previously passed on land and its motions 

 had been running and leaping, it would now pass 

 its life in water, and its chief motions would be 

 swimming and diving. A different set of muscles 

 would be called into play ; there woqld be a differ- 



