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 Muller, 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 



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. 



