NATURAL SELECTION 231 



forms, if differing very little, are generally ranked as varieties, not- 

 withstanding that they cannot be closely connected; but the amount 

 of difference considered necessary to give to any two forms the rank 

 of species cannot be defined. • In genera having more than the average 

 number of species in any country, the species of these genera have 

 more than the average number of varieties. In large genera the species 

 are apt to be closely, but unequally, allied together, forming httle 

 clusters round other species. Species very closely allied to other 

 species apparently have restricted ranges. In all these respects the 

 species of large genera present a strong analogy with varieties. And 

 we can clearly understand these analogies, if species once existed as 

 varieties, and thus originated; whereas, these analogies are utterly 

 inexplicable if species are independent creations. 



We have, also, seen that it is the most flourishing or dominant 

 species of the larger genera within each class which on an average yield 

 the greatest number of varieties; and varieties, as we shall hereafter 

 see, tend to become converted into new and distinct species. Thus 

 the larger genera tend to become larger; and tliroughout nature the 

 forms of life which are now dominant tend to become still more domi- 

 nant by leaving many modified and dominant descendants. But by 

 steps hereafter to be explained, the larger genera also tend to break up 

 into smaller genera. And thus, the forms of life throughout the uni- 

 verse become divided into groups subordinate to groups. 



THE TERM "STRUGGLE FOR EXISTENCE" USED IN A LARGE SENSE 



I should premise that I use this term in a large and metaphorical 

 sense including dependence of one being on another, and including 

 (which is more important) not only the life of the individual, but 

 success in leaving progeny. Two canine animals, in a time of dearth, 

 may be truly said to struggle with each other which shall get food and 

 live. But a plant on the edge of a desert is said to struggle for life 

 against the drought, though more properly it should be said to be 

 dependent on the moisture. A plant which annually produces a 

 thousand seeds, of which only one of an average comes to maturity, 

 may be more truly said to struggle with the plants of the same 

 and other kinds which already clothe the ground. The mistletoe is 

 dependent on the apple and a few other trees, but can only in a 

 far-fetched sense be said to struggle with these trees, for, if too many 

 of these parasites grow on the same tree, it languishes and dies. But 

 several seedling mistletoes, growing close together on the same branch, 



