326 ORIGIN OF SPECIES 



with yellow and white varieties of a distinct species, more 

 seed is produced by the crosses between the similarly coloured 

 flowers, than between those which are differently coloured. 

 Mr. Scott also has experimented on the species and varieties 

 of Verbascum; and although unable to confirm Gartner's 

 results on the crossing of the distinct species, he finds that 

 the dissimilarly coloured varieties of the same species yield 

 fewer seeds, in the proportion of 86 to loo, than the similarly 

 coloured varieties. Yet these varieties differ in no respect 

 except in the colour of their flowers ; and one variety can 

 sometimes be raised from the seed of another. 



Kolreuter, whose accuracy has been confirmed by every 

 subsequent observer, has proved the remarkable fact, that 

 one particular variety of the common tobacco was more 

 fertile than the other varieties, when crossed with a widely 

 distinct species. He experimented on five forms which are 

 commonly reputed to be varieties, and which he tested by 

 the severest trial, namely, by reciprocal crosses, and he found 

 their mongrel offspring perfectly fertile. But one of these 

 five varieties, when used either as the father or mother, and 

 crossed with the Nicotiana glutinosa, always yielded hybrids 

 not so sterile as those which were produced from the four 

 other varieties when crossed with N. glutinosa. Hence the 

 reproductive system of this one variety must have been 

 in some manner and in some degree modified. 



From these facts it can no longer be maintained that var- 

 ieties when crossed are invariably quite fertile. From the 

 great difficulty of ascertaining the infertility of varieties in 

 a state of nature, for a supposed variety, if proved to be in- 

 fertile in any degree, would almost universally be ranked as 

 a species ; — from man attending only to external characters 

 in his domestic varieties, and from such varieties not hav- 

 ing been exposed for very long periods to uniform conditions 

 of life ; — from these several considerations we may conclude 

 that fertility does not constitute a fundamental distinction 

 between varieties and species when crossed. The general 

 sterility of crossed species may safely be looked at, not as a 

 special acquirement or endowment, but as incidental on 

 changes of an unknown nature in their sexual elements. 



