118 STUDIES IN ANIMAL LIFE. 



explained through, inheritance, and the complex 

 action of natural selection entailing extinction and 

 divergence of character. The affinities of all the 

 beings of the same class have sometimes been rep- 

 resented by a great tree. I believe this simile 

 largely speaks the truth. The green and budding 

 twigs may represent existing species, and those pro- 

 duced during each former year may represent the 

 long succession 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 sur- 

 rounding twigs and branches, in the same manner 

 as species and groups of species have tried to over- 

 master other species in the great struggle for life. 

 The limbs divided into great branches, and these 

 into lesser branches, were themselves once, when 

 the tree was small, budding twigs; and this con- 

 nection of the former and present buds by ramify- 

 ing branches may well represent the classification 

 of all extinct and living species in groups subordi- 

 nate to groups. Of the many twigs which flourish- 

 ed when the tree was a mere bush, only two or 

 three, now grown into great branches, yet survive 

 and bear all the other branches ; so with the spe- 

 cies which lived during long-past geological peri- 

 ods, very few now have living and modified de- 

 scendants. ... As buds give rise by growth to 

 fresh buds, and these, if vigorous, branch out and 

 overtop on all sides many a feebler branch, so by 

 generation, I believe, it has been with the great 



