Chap. IV. 
SUMMAEY. 
129 
lies, families, orders, sub-classes, and 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 spe¬ 
cies 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 sf)ecies 
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 connexion 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 fiou- 
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 has decayed 
and dropped off; and these lost branches of various 
G 3 
