94 MR. C. DARWIN ON THE DIMORPHIC CONDITION 
lips, but further experiments are absolutely necessary. We may 
also suspect that the fact noticed by florists*, that the varieties 
of the Polyanthus never come true from seed, may be in part 
due to their habitually crossing with other varieties of the Poly- 
anthus. 
The simple fact of two individuals of the same undoubted species, 
when homomorphically united, being as sterile as are many distinct 
species when crossed, will surprise those who look at sterility as a 
special endowment to keep created species distinct. Hybridizers 
have shownt that individual plants of the same species vary in 
their sexual powers, so far that one individual will unite more 
readily than another individual of the same species with a distinct 
species. Seeing that we thus have a groundwork of variability in 
sexual power, and seeing that sterility of a peculiar kind has been 
acquired by the species of Primula to favour intercrossing, those 
who believe in the slow modification of specific forms will natu- 
rally ask themselves whether sterility may not have been slowly 
acquired for a distinet object, namely, to prevent two forms, whilst 
being fitted for distinct lines of life, becoming blended by marriage, 
and thus less well adapted for their new habits of life. But many 
great difficulties would remain, even if this view could be main- 
tained. 
Whether or not the dimorphic condition of the Primule has 
any bearing on other points in natural history, it is valuable as 
showing how nature strives, if I may so express myself, to favour 
the sexual union of distinct individuals of the same species. The 
resources of nature are illimitable; and we know not why the 
species of Primula should have acquired this novel and curious aid 
for checking continued self-fertilization through the division of the 
individuals into two bodies of hermaphrodites with different 
sexual powers, instead of by the more common method of the 
separation of the sexes, or by the maturity of the male and female 
elements at different periods, or by other such contrivances. Nor 
do we know why nature should thus strive after the intercrossing 
of distinct individuals. We do not even in the least know the final 
cause of sexuality; why new beings should be produced by the union 
of the two sexual elements, instead of by a process of partheno- 
genesis. When we look to the state in which young mammals 
and birds are born, we can at least see that the object gained is 
* Mr. D. Beaton, in * Journal of Horticulture, May 28, 1861, pp. 154, 244. 
+ Gartner, Bastarderzeugung, s. 165. 
