260 THE ORIGIN OF SPECIB.S 



first sight exactly the reverse of what occurs, in the ordinary 

 unions of the individuals of the same species and with crosses 

 between distinct species. It is, however, doubtful whether this is 

 really so; but I will not enlarge on this obscure subject. 



We may, however, infer as probable from the consideration of 

 dimorphic and trimorphic plants, that the sterility of distinct 

 species when crossed and of their hybrid progeny, depends ex- 

 clusively on the nature of their sexual elements, and not on any 

 difference in their structure or general constitution. We are also 

 led to this same conclusion by considering reciprocal crosses, in 

 which the male of one species cannot be united, or can be united 

 with great difficulty, with the female of a second species, while 

 the converse cross can be effected with perfect facility. That 

 excellent observer, Gartner, likewise concluded that species when 

 crossed are sterile owing to differences confined to their reproduc- 

 tive systems. 



FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL 

 OFFSPRING, NOT UNIVERSAL 



It may be urged as an overwhelming argument that there 

 must be some essential distinction between species and varieties, 

 inasmuch as the latter, however much they may differ from each 

 other in external appearance, cross with perfect facility, and 

 yield perfectly fertile offspring. With some exceptions, presently 

 to be given, I fully admit that this is the rule. But the subject 

 is surrounded by difficulties, for, looking to varieties produced 

 under nature, if two forms hitherto reputed to be varieties be 

 found in any degree sterile together, they are at once ranked 

 by most naturalists as species. For instance, the blue and red 

 pimpernel, which are considered by most botanists as varieties, 

 are said by Gartner to be quite sterile when crossed, and he 

 consequently ranks them as undoubted species. If we thus argue 

 in a circle, the fertility of all varieties produced under nature 

 will assuredly have to be granted. 



If we turn to varieties, produced, or supposed to have been 

 produced, under domestication, we are still involved in some 

 doubt. For when it is stated, for instance, that certain South 

 American indigenous domestic dogs do not readily unite with 

 European dogs, the explanation which will occur to every one, 

 and probably the true one, is that they are descended from 

 aboriginally distinct species. Nevertheless the perfect fertility 

 of so many domestic races, differing widely from each other in 



