Page - 143 - in The Origin of Species

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