244 CLASSICS OF MODERN SCIENCE 

 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 overm.astered 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 repre- 

 sent the classification of all extinct and living species in groups sub- 

 ordinate 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 straggling branch springing from a fork low down in a 

 tree, and which by some chance has been favoured and is still alive 

 on its summit, so we occasionally see an animal like the Ornithorhyn- 

 chus or Lepidosiren, which in some small degree connects by its af- 

 finities two large branches of life, and which has apparently been 

 saved from fatal competition by having inhabited a protected 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 branches the crust of the earth, and 

 covers the surface with its ever-branching and beautiful ramifications. 



