NATURAL SELECTION 117 



classes. The several subordinate groups in any class 

 cannot be ranked in a single file, but seem rather to be 

 clustered round points, and these round other points, 

 and so on in almost endless cycles. On the view that 

 each species has been independently created, I can see 

 no explanation of this great fact in the classification of 

 all organic beings ; but, to the best of my judgment, 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 

 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 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 surrounding 

 twigs and branches, in the same manner as species and 

 groups of species have tried to overmaster 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 small, 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 flou- 

 rished 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 species which 

 lived during long-past geological periods, very few now 

 have living and modified descendants. From the first 

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

 and dropped off; and these lost branches of various 

 size* may represent those whole orders, families, and 

 genera which have now no living representatives, and 

 which are known to us only from having been found in 

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

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



