ACQUIRED VARIATIONS 317 
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 ot 
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. 
