256 FERTILITY OF VARIETIES Chap. VIII. 



certain -ways. With dimorpliic plants, tlie unions between the 

 two distinct forms arc alone quite fertile, and jmxluce quite 

 fertile ofl'spring-, while unions between individuals belonging 

 to the same form are more or less sterile ; so that the result is 

 exactly the reverse of what occurs with distinct sj)ecies. With 

 dimorphic plants the resultant sterility is quite indejiendent of 

 any difference in general structure or constitution, for it arises 

 from the union of individuals belonging not only to the same 

 species, but to the same form. It must, therefore, depend on 

 the nature of the sexual elements, which are so adapted to each 

 other, that the male and female elements occurring in the same 

 form do not suit each other, Avhile those occurring in the two 

 distinct forms are mutually suited to each other. From these 

 considerations, it seems proljable that the sterility of distinct 

 species when crossed, and of their hybrid progeny, dejiends ex- 

 clusively on the nature of their sexual elements, and not on 

 any general difference in structure or constitution. We are, 

 indeed, led to this same conclusion by considering reciprocal 

 crosses, in which the male of one species cannot be imited, or 

 can be imited with great dilhculty, with the female of a sec- 

 ond species, while the converse cross can be effected with per- 

 fect facility ; for this difference in the facility of making recip- 

 rocal crosses and in the fertility of their offspring must be at- 

 tributed cither to the male or to the female element in the first 

 species having been differentiated, with reference to the sexual 

 elements of the second species in a higher degree than in the 

 converse case. That excellent observer, Gartner, likewise 

 came to this same conclusion, namely, that species when 

 crossed are sterile owing to differences confined to their repro- 

 ductive systems. 



Fertility of Varieties tchen crossed, and of their Mongrel 



Offqyring. 



It may be urged, as an overwhelming argument, that there 

 must be some essential distinction between species and varie- 

 ties, inasmuch as the latter, however nuich tliey may differ 

 from each other in external ajipearance, cross with perfect facil- 

 it>-, and yield perfectly fertile offspring. Witli some excep- 

 tions, presently to be given, I fully admit that this is the rule. 

 But the subject is surrounded by difliculties, for, looking to va- 

 rieties, produced under nature, if two forms hitherto reputed to 

 be varieties be found in any degree sterile together, they are 



