582 THE BEGINNINGS OF LIFE. 



the hairy coverings of sheep, goats, dogs, cats, horses, 

 and other animals 1 . 



Many of such instances cited by Mr. Darwin and 

 other writers, abundantly testify to the fact that 

 changes are frequently induced, both amongst animals 

 and plants, by alterations in the conditions of life. 

 And these changes are of course liable to become 

 intensified in the course of time. On the other hand, 

 we know quite well that other animals and plants, 

 which happen to have a less impressionable consti- 

 tution, may range over large portions of the world 

 without undergoing any appreciable modification 2 . 



Internal Causes of Change and ' Progressive 

 Development. 3 



The divergence of opinion to which we have already 

 referred is, however, as nothing compared to that 

 which obtains concerning the existence or non-exist- 

 ence of certain 'principles' or tendencies inherent in 

 the organism itself, and leading, in the main, to an 

 increasing complexity of organization. 



The existence of such 'principles' or tendencies 

 has been frequently affirmed, though even more fre- 

 quently denied, either implicitly or explicitly. 



1 'Animals and Plants under Domestication,' vol. ii. p. 278. 



2 Similarly we find that the majority of crystals may be exposed to the 

 incidence of a considerable change in conditions, without producing 01 

 necessitating molecular rearrangements, though others, such as mercuric 

 iodide, are most sensitive to changing influences, under which thej 

 are apt to assume not only changes of form but also complete altera- 

 tions of colour. (See p. 82.) 



