ENVIEONMBNT AMONG ANIMALS AND PLANTS 335 



species as living under a more or less clearly defined environment 

 into which very many elements enter ; whether the variations 

 in these elements are great or small, there is an average condition, 

 and to that average condition a certain type of germinal con- 

 stitution gives the best response. In the case of a free-living 

 animal in the adult form the germinal constitution does not 

 respond readily to ordinary variations from the normal, though 

 it has to be remembered that extreme influences such as those 

 caused by parasites may cause marked reactions. Further, 

 it has always to be borne in mind that during development 

 all animals and plants are particularly susceptible to environ- 

 mental changes. It is not possible to make any precise 

 statement as to how far the differences we see under natural 

 conditions are modifications and how far mutations. All that 

 we can say is that the part played by the environment in producing 

 modifications is on the whole smaller in the case of free-living 

 animals than in the case of sessile animals and of plants. 



