SUMMARY. 135 



of extinct species. At each period of growth all the grow- 

 ing twigs have tried to branch out on all sides, and to over- 

 top 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 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. Prom 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, 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 of life, and which has apparently been 

 saved from fatal competition by having inhabited a pro- 

 tected station. As buds give rise by growth to fresh buds, 

 and these, if vigorous, branch out and overtop on all sides 

 many a feebler branch, so by generation I believe it has 

 been with the great Tree of Life, which fills with its dead 

 and broken bi'anches the crust of the earth, and covers the 

 surface with its ever-branching and beautiful ramifications. 



