148 Evolution and Adaptation 
In striking contrast to the sterility between species is the 
fertility of varieties. If, as Darwin believes, varieties are. 
incipient species, we should certainly expect to find them 
becoming less and less fertile with other fraternal varieties, or 
with the parent forms in proportion as they become more 
different. Yet experience appears to teach exactly the op- 
posite ; but the question is not a simple one, and the results 
are not so conclusive as appears at first sight. Let us first 
see how Darwin met this obvious contradiction to his view. 
In the first place, he points out that all species are not in- 
fertile when crossed with other species. The sterility of 
various species, when crossed, is so different in degree, and 
graduates away so insensibly, and the fertility of pure species 
is so easily affected by various circumstances, that it is most 
difficult to say where perfect fertility ends and sterility be- 
gins. “It can thus be shown that neither sterility nor fer- 
tility afford any certain distinction between species and 
varieties.” Darwin cites several cases in plants in which 
crosses between species have been successfully accomplished. 
The following remarkable results are also recorded: ‘ Indi- 
vidual plants in certain species of Lobelia, Verbascum, and 
Passiflora can easily be fertilized by pollen from a distinct 
species, but not by pollen from the same plant, though this 
pollen can be proved to be perfectly sound by fertilizing 
other plants or species. In the genus Hippeastrum, in Co- 
rydalis as shown by Professor Hildebrand, in various orchids 
as shown by Mr. Scott and Fritz Miiller, all the individuals 
are in this peculiar condition. So that with some species, 
certain abnormal individuals, and in other species all the 
individuals, can actually be hybridized much more readily 
than they can be fertilized by pollen from the same individual 
plant!” 
1 A somewhat parallel case has recently been discovered by Castle for the her- 
maphroditic ascidian Ciona intestinalis. In this case the spermatozoa of any 
individual fail to fertilize the eggs of the same individual, although they will fer- 
tilize the eggs of any other individual. 
