222 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
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 little 
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 throughout 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, 
