RELATIONSHIP OF SPECIES. 29 



inter se, and breeding easily and indiscriminately with the 

 parent species.") 



B. Animals producing young more or less fertile. 

 Examples : I was at that time only able to presume perhaps 



Chrysomitris and Serinus, but suggested that reports one heard of 

 Mules breeding might serve as examples ; since then I have seen 

 that Broca had described several graduations from " Para- 

 genesic" (with a "partial fecundity") to "Dysgenesic," "nearly 

 altogether sterile " (i. e., infertile with each other, but some- 

 times rarely breeding with one of the parent species, these 

 three-quarters young being infertile) : Broca's examples were 

 chiefly drawn, and perhaps not very correctly, from various 

 human hybrids. Another example, however, is Pigeon and 

 Collared Dove (Suchetet). 



C. Animals producing sterile young. 



Examples: Lion and Tiger, Horse and Burchell's Quagga 

 (which I only then included as " probable "), Duck and Muscovy, 

 Blackcock and Pheasant, Serinus and Carduelis, Serinus and 

 Ligurinus, Serinus and Alario, &c. This degree, " entirely 

 infertile," Broca termed " Aganesic." 



D. Animals mating, but not always producing young. 

 Examples: Horse and Ass (nearly always), Carduelis and 



Pyrrhida, Ligurinus and Carduelis (frequently), Serinus and 

 Pyrrhula (very rarely). 



Broca does not specify this stage. Lest it should cause 

 surprise, I may state that I have included Horse and Ass here on 

 the authority of Tegetmeier and Sutherland,* who state some 

 mares quite capable of producing young to a Horse do not do so 

 to an Ass. (I have heard, however, of vice versa results being 

 obtained.) 



E. Animals mating, but not producing young at all. 

 Examples : Sheep and Goat, Eland and Cow (?), Buffalo and 



Cow (?), Fringilla and Serinus, Seedeater (Crithagra?) and Serinus. 



These results, meagre though they be (I could name several 

 more, however), seem to my mind to apply very correctly to the 

 degree of relationship of the contracting parties. 



In stage A. all these contracting parties seem to have very 

 many if not entirely corresponding congeneric actions, or "in- 



* ' Horses, Asses, Zebras, Mules, and Mule Breeding' (1895). 



