SPECIES AND THEIR ORIGIN 307 



the race. In the nature of things, direct evidence 

 for such an hypothesis is not abundant, but the in- 

 ferential evidence is overwhelming, and one can 

 hardly work with natural objects at first hand with- 

 out being constantly impressed with it. Since 

 life does not exist apart from living organisms, and 

 since organisms are always found in the aggregates 

 we call species, the question of the evolution of 

 life is wrapped up in that of the evolution of species, 

 and the origin of new species is its central problem. 



EVIDENCE FOR THE EVOLUTION OF SPECIES 

 IN THE PAST 



The age of the earth is very great, and for only a 

 fraction of its life has it been habitable for living 

 organisms. Yet, looking back from the present day, 

 an enormous vista of time opens up during which we 

 find evidences in the rocks of the existence of animal 

 and plant forms, the great majority of which are now 

 extinct. Only the scattered fragments have been 

 preserved. 1 When, however, we piece together this 



1 "The affinities of all the beings of the same class have sometimes 

 been represented by a great tree. I believe this simile largely speaks 

 the truth. The green and budding twigs may represent existing species ; 

 and those produced during former years may represent the long succes- 

 sion of extinct species. At each period of growth all the growing twigs 

 have tried to branch out on all sides, and to overtop and kill the surround- 

 ing twigs and branches, in the same manner as species and groups of 

 species .have at all times overmastered other species in the great battle 

 for life. The limbs, divided into great branches, and these into lesser 

 and lesser branches, were themselves once, when the tree was young, 

 budding twigs ; and this connection of the former and present buds by 



