AND TBIMOBPniSM. 301 



ditions, and yet be sterile when united in certain ways. 

 For we must remember that it is the union of the sexual 

 elements of individuals of the same form, for instance, of 

 two long-styled forms, which results in sterility; while it 

 is the union of the sexual elements proper to two distinct 

 forms which is fertile. Hence the case appears at first sight 

 exactly the reverse of wliat 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 considera- 

 tion of dimorphic and trimorphic plants, that the sterility 

 of distinct species when crossed and of their hybrid pro- 

 geny, depends exclusively 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 con- 

 clusion 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 con- 

 cluded that species when crossed are sterile owing to dif- 

 ferences confined to their reproductive systems. 



FBKTILITT OF VAEIBTIBS VfHBiq- CKOSSBD, AND OF THBIE 

 MONOEBL OFFSPRISTG, NOT UNIYBKSAL. 



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 



