SUMMARY OF CHAPTER 143 



most closely related, species of the same genus less closely 

 and unequally related, forming sections and sub-genera, spe- 

 cies of distinct genera much less closely related, and genera 

 related in different degrees, forming sub-families, families, 

 orders, sub-classes and classes. The several subordinate 

 groups in any class cannot be ranked in a single file, but 

 seem clustered round points, and these round other points, 

 and so on in almost endless cycles. If. species had been in- 

 dependently created, no explanation would have been pos- 

 sible of this kind of classification; but it is explained through 

 inheritance and the complex action of natural selection, en- 

 tailing extinction and divergence of character, as we have 

 seen illustrated in the diagram. 



The affinities of all the beings of the same class have some- 

 times 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 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 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 

 ramifying branches may well represent the classification of 

 all extinct and living species in groups subordinate to groups. 

 Of the many twigs which flourished when the tree was a 

 mere bush, only two or three, now grown into great branches, 

 yet survive and bear the other branches ; so with the species 

 which lived during long-past geological periods, very few 

 have left living and modified descendants. From the first 

 growth of the tree, many a limb and branch has decayed and 

 dropped off; and these fallen branches of various sizes may 

 represent those whole orders, families, and genera which have 

 now no living representatives, and which are known to us 

 only in a fossil state. As we here and there see a thin strag- 

 gling branch springing from a fork low down in a tree, and 

 which by some chance has been favoured and is still alive on 



