SUMMARY. 117 



intermediate forms of life. On these principles, the nature 

 of the affinities, and the generally well defined distinctions 

 between the innumerable organic beings in each class through- 

 out the world, may be explained. It is a truly wonderful 

 fact — the wonder of which we are apt to overlook from famil- 

 iarity — that all animals and all plants, throughout all time 

 and space, should be related to each other in groups, subordi- 

 nate to groups, in the manner which we everywhere behold 

 - — namely, varieties of the same species most closely related, 

 species of the same genus less closely and unequally related, 

 forming sections and sub-genera, species 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 independently created, 

 no explanation would have been possible of this kind of 

 classification ; but it is explained through inheritance and 

 the complex action of natural selection, entailing 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. T 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 j and these fallen branches of various 



