ACQUIRED VARIATIONS 319 



animals, like plants, exhibit. No necessity for definite 

 and co-ordinated movements involving their whole 

 structure forces the development of these animals 

 along certain definite paths. External circumstance 

 is, therefore, free to mould them into a host of slightly 

 different shapes. And thus the great variability of 

 the species of corals, for instance, is doubtless deter- 

 mined to a large extent by the influence of different 

 environmental conditions. 



Strictly speaking, the term variability ought not to 

 be applied to modifications of this description. It will, 

 perhaps, be most convenient, however, to distinguish 

 true variations having their origin in differences 

 among the germ-cells as genetic variations, contrasting 

 them with the acquired variations which arise during 

 the development of individuals. 



Enough has now been said to show that it is a very 

 difficult matter to distinguish in the case of continuous 

 variations between those which are genetic and those 

 which are acquired. 



It is easy to understand how acquired variations 

 come to be continuous, and to obey the law of normal 

 variability. We saw that the normal distribution of 

 characters was induced by the random operation of a 

 multitude of small causes. During the development 

 of the individual a great number of different external 

 influences come into play, leading to slight modifications 

 of every part, now in one direction, now in another. 

 This being so, we may be quite sure that a large pro- 

 portion of the normal variability which any species 

 exhibits is acquired. 



