NATUEAL CONDITIONS OF EXISTENCE 



AS THEY AFFECT 



ANIMAL LIFE. 



INTEODUOTION. 



No one at the present day disputes the fact that the Darwinian 

 theory has exerted an extensive influence not only on the develop- 

 ment of the natural sciences, but in other branches of study ; it 

 would be superfluous here to bring forward any proofs of this. 

 It is equally recognised that it is to this influence that modern 

 zoology owes its most essential pretensions to be regarded as 

 of equal estimation with other sciences. But it may be advis- 

 able to pause for a moment at the question, In what way is it 

 that this influence has afiected zoology? since in this book we 

 have to deal exclusively with this science. 



Darwin showed the possibility of discovering the path 

 which nature struck out in order to produce her endless variety 

 of animal forms, and of detecting the means she has employed 

 in her task. Hence first arose those efibrts, so natural in the 

 zoologist, to acquire some comprehension of the succession in time 

 of the difierent types in the animal kingdom, since all who 

 recognise Darwin's teaching must regard it not as an arbitrary 

 and lawless assemblage of independent species, but, on the 

 contrary, as a great family of organisms of which the individual 

 members, whether living or extinct, are united by a real, 



