188 THE ORIGIN OF SPECIES 



The afliuities of all the beings of the same class have 

 sometimes been represented by a great tree. I believe 

 tiiis 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 surrounding 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 flour- 

 ished 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 repre- 

 sent 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 

 straggling branch springing from a fork low down in a 

 tree, and which by some chance has been favored and is 

 still alive on its summit, so we occasionally see an animal 

 like the Ornithorhynchus or Lepidosiren, which in some 

 small degree connects by its affinities two large branches 



