204 THE STORY OF BIRD-LIFE. 



died in their endeavour to overcome these 

 vicissitudes, so the dead point to the living to 

 show by what means they have managed to 

 survive. The living are the adaptable descen- 

 dants, who have succeeded in shaking off certain 

 of those family traditions and family idiosyn- 

 crasies which stood in the way of advancement, 

 or even of continued existence. Advancement 

 followed as a result of ability to obey. Thus 

 we get the evolution of a species. 



It must be mentioned here, however, that it 

 becomes a very difficult matter to say exactly 

 Avhat is a species. It is obvious that if a species 

 is the outcome of a gradual change from one 

 form to another, from a short to a long-billed 

 bird for instance, then if we had every member 

 in the series alive before us, it would be impos- 

 sible to say where the short-bill species ceased 

 and the long-billed began. Yet the lowest and 

 the highest members in the series would be 

 easily distinguished. The "species" living at 

 the present day are isolated links in such a 

 chain of individuals, recognisable as distinct 

 because they are isolated. Similarly we know 

 that if we had many of the extinct forms for 

 comparison, the differences between the principal 

 larger existing groups would fade away ; and we 

 should have a difficulty in saying, for instance, 

 where the lion begins and the ox ends. For 

 strange as it may seem to many, we have enough 

 evidence to prove that the carnivora and the 

 herbivora are traceable to one common ancestor. 



With the birds, if we could go far enough 

 back, and if we had all the links in the chain. 



