184 



HYBRIDISM. 



Chap. XIX. 



important, because they show us, firstly, that the physiological 

 x - - x of lessened fertility, both in first crosses and in hybrids is 



no safe criterion of specific distinctio 



secondly, becaus 



their 



may conclude that there must be some unknown law or bond 



ng the infertility of illegitimate unions with that of 

 egitimate offspring, and we are thus led to extend this 

 view to first crosses and hybrids ; thirdly, because we find, and 

 this seems to me of especial importance, that with triniorphic 

 plants three forms of the same species exist, which when crossed 



inner are infertile, and yet these forms differ 



particular m 

 in no respect from each other. 



'& 



as in the relative length of 



in their 



product 



the size, form, and colour of the pollen 



stamens and pistils 



ture of the stigma, and in the number and size of the seeds 

 With these differences and no others, either in organisation 01 

 constitution, we find that the illegitimate unions and the ille- 

 gitimate progeny of these three forms are more or less sterile 

 and closely resemble in a whole series of relations 



the first 



unions and hybrid offspring of distinct species. From this we 

 may infer that the sterility of species when crossed and of their 

 hybrid progeny is likewise in all probability exclusively due 

 to differences confined to the reproductive system. We" have 

 indeed been brought to a similar conclusion by observing that 

 the sterility of crossed species does not strictly coincide with 

 their systematic affinity, that is, with 

 resemblances ; 



general constiti 



the sum of their exter 

 nor does it coincide with their similarity 



But 



are more especially led 



by considering reciprocal crosses, in which the 



male of one species cannot be united 



or can be 



ted 



difficulty 



female of a second species 



the converse cross can be effected with perfect facility; for 

 this difference in the facility of making reciprocal crosses, and 

 in the fertility of their offspring, must be attributed either to 

 the male or female element in the first species having been 

 differentiated with reference to the sexual ^m^t nftbe second 



species 



higher 



degree than in the converse case. In so 

 complex a subject as Hybridism it is of considerable importance 



i, namely, that the 



thus to arrive at a definitiv 



sterility which almost 



lusio 



ably folio 



ws 



of 



