CHAP. IX.] AND TRIMORPHISM. 239 



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, whilst 

 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 repro- 

 ductive 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 

 appearance, for instance those of the pigeon, or of the cabbage, 

 is a remarkable fact ; more especially when we reflect how many 

 species there are, which, though resembling each other most 

 closely, are utterly sterile when intercrossed. Several considera* 

 tions, however, render the fertility of domestic varieties lesa 



