CiiAF. IV. SUMMARY. I35 



we see proof by looking to the inhabitants of any small spot 

 or (o naturalized productions. Therefore, durin> the modifica- 

 tion of the descendants of anj- one species, and dm'in<>* the in- 

 cessant stru^f^le of all species to increase in numbers, the more 

 diversified these descendants become, tlic better will be their 

 chance of succccdinrr in the battle for life. Thus, the small 

 dill'erences distin<]^uishint^ varieties of the same species, steadily 

 tend to increase, till they come to equal the greater diflfereuces 

 between species of tlie same genus, or even of distinct genera. 



We have seen that it is the common, the widely-dilfused, 

 and widely-ranging species, belonging to the larger genera 

 within each class, which vary most; and these tend to transmit 

 ((> their modified offspring that superiority which now makes 

 them dominant in their own countries. Natural selection, as 

 has just been remarked, leads to divergence of character and to 

 much Extinction of the less improved and intermediate forms of 

 life. On these principles, the nature of the affinities, and the 

 generally AvoU-dellned distinctions of the innumerable organic 

 beings in each class througliout the Avorld, may be explained. 

 It is a truly wonderful fact — the wonder of M'hich we are apt 

 to overlook from familiarity — that all animals and all plants 

 tliroughout all time and space should be related to each other 

 in natural groups subordinate to groups, in the manner 

 which Ave everywhere behold — namely, varieties of the same 

 species most closely related together ; species of the same genus 

 less closely and unequally related together, forming sections 

 and sub-genera ; species of distinct genera much less closely 

 related, and genera related m difierent degrees, forming sub- 

 families, families, orders, sub-classes, and classes. The several 

 subordinate groups in any class cannot be ranked in a single 

 lile, but seem rather to be clustered round points, and these 

 round other points, and so on in almost endless cycles. If 

 each species has been independently created, no explanation 

 rm be given of this great fact in the classification of all organic 

 beings; but it is explained through inheritance and tlie com- 

 plex action of natural selection, entailing extinction and diver- 

 gence of character, as we have seen illustrated in the diagram. 



The affinities of all the beings of the same class have some- 

 limes been repres(Mited by a great tree. I believe this simile 

 largely speaks (lie trulli. 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 



