322 ORIGIN OF SPECIES 



hybrids, is no safe criterion of specific distinction; secondly, 

 because we may conclude that there is some unknown bond 

 which connects the infertility of illegitimate unions with 

 that of their illegitimate offspring, and we are led to extend 

 the same view to first crosses and hybrids; thirdly, because 

 we find, and this seems to me of especial importance, that 

 two or three forms of the same species may exist and may 

 differ in no respect whatever, either in structure or in con- 

 stitution, relatively to external conditions, 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; whilst 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 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 dis- 

 tinct species when crossed and of their hybrid progeny, de- 

 pends exclusively on the nature of their sexual elements, and 

 not on any difference in their structure or general constitu- 

 tion. 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, Gart- 

 ner, likewise concluded that species when crossed are sterile 

 owing to differences confined to their reproductive 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 vari- 

 eties, 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 



