HYBRIDISM 39 



plants are important, because they show us, first, that 

 the physiological test of lessened fertility, both in first 

 crosses and in 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 off- 

 spring, 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 constitu- 

 tion, 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; 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 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 con- 

 sideration of dimorphic and trimorphic plants, that the 

 sterility of distinct species when crossed, and of their 

 hybrid progeny, 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 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 



— SCIENCK — 19 



