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CLASSIFICATION OF FI A NTS 
forms by aid of natural selection of continuous variations. 
Let us suppose that at a certain period there are five species, 
represented by the letters A to E, living in a given region, 
and that A and D are represented by many, B, C, and E by 
few, individuals. Take first the case of A. The offspring of 
the various individuals will form a large number, represented 
by the letters a 1 ...h 1 . They will vary in their characters. 
Suppose a 1 to represent those individuals which possess in 
the most marked degree a certain character or (more usually) 
sum of characters which in the struggle for existence will be 
advantageous to them. Suppose k x to represent another 
group possessing in a high degree some other beneficial 
character ; the intermediate groups possess these characters 
in a less marked degree. Then, in the struggle for existence, 
the tendency will be for the groups a 1 and k x to defeat the 
intermediate groups and also the parent form . Probably the 
groups a l k x will not be the only survivors, but they will pro- 
duce most offspring. In the next generation the offspring 
will again vary in the same characters and those that vary 
furthest in favourable directions, i.e. the groups a 2 k 2 , will 
tend to produce most offspring. The same process may be 
repeated in every generation, and thus the original species 
will give rise to steadily diverging lines of offspring (the 
divergence is not shown in the diagram beyond the second 
generation). After many generations, the differences between 
the group a and the group k , at first infinitesimal, will 
become obvious, and we may now say that the species A 
has two varieties , a and k. The type form A itself will per- 
haps usually have ceased to exist in this locality, but if one 
of the varieties to which it has given rise be much commoner 
than the others, naturalists generally, though incorrectly, term 
this the ‘type’ of the species, unless, as probably often 
happens, the new variety or varieties have arisen on the 
margin of the area of the earth’s surface occupied by the 
parent species, which may then continue to exist side by side 
with the variety. 
Varieties are common in most of the larger genera and 
species, e.g. Rosa, Rubus, Hieracium, Salix. They differ 
from one another chiefly in small and variable characters, 
especially of the vegetative organs ; e.g. there are two 
