Distinction Bct-i^ccn Species and Varieties. 583 
We will now discuss the principle illustrated by this 
instance from a more general point of view. In the 
literature of the subject we frequently find the opinion 
that forms which are mutually fertile and produce a 
normal harvest of seed, giving fertile hybrids, are to be 
regarded as varieties of one and the same species. Forms, 
however, the union of which is followed by a diminution 
in fertility and the hybrids of which are less fertile than 
the species crossed, are regarded by the majority of the 
investigators as specifically distinct. These generaliza- 
tions have served as criteria of relationship from the 
time of KOLREUTER and GARTNER up to the present ; and 
DARWIN himself relies on them in considerations of this 
kind. 1 Based as they were on extensive experience and 
on a profound systematic knowledge, they constitute prin- 
ciples which bid fair to become universally recognized. 
For these reasons they deserve to be placed in the fore- 
ground as a convenient point of departure for our dis- 
cussion : and our object will be not to find out their weak 
points or to replace them by others, but rather to give 
them the more definite forms required by our present 
knowledge of hybridization. 
Therefore we will start from the oft-cited proposi- 
tion that varieties are only small species. 2 This means 
that the difference between species and varieties is not 
of a fundamental nature but rather of a gradual or even 
a conventional kind. Moreover we will start from the 
conception enunciated in the first part of this volume, 
according to which the forms which compose the collec- 
tive species are mainly of two kinds (p. 64) : Hoinonoin- 
See FOCKE, Die Pflarizeninisclilingc, pp. 436, 446-502, etc. 
2 See Vol. T, p. i/i and above p. 580. 
