88 DOMESTICATION ELIMINATES STEKILITY. Chap. XVI. 



briefly discuss the conjoint bearing of this fact, and others, 

 on the difference in fertility between crossed varieties and 

 crossed species. 



Domestication eliminates the tendency to Sterility which is general 

 with Species when crossed. 



This hypothesis was first propounded by Pallas, 28 and has 

 been adopted by several authors. I can find hardly any 

 direct facts in its support ; but unfortunately no one has 

 compared, in the case of either animals or plants, the fertility 

 of anciently domesticated varieties, when crossed with a 

 distinct species, with that of the wild parent-species when 

 similarly crossed. No one has compared, for instance, the 

 fertility of Gallus bankiva and of the domesticated fowl, when 

 crossed with a distinct species of Gallus or Phasianus ; and 

 the experiment would in all cases be surrounded by many 

 difficulties. Dureau de la Malle, who has so closely studied 

 classical literature, states 29 that in the time of the Eomans 

 the common mule was produced with more difficulty than at 

 the present day ; but whether this statement may be trusted 

 I know not. A much more important, though somewhat dif- 

 ferent, case is given by M. Groenland, 30 namely, that plants, 

 known from their intermediate character and sterility to 

 be hybrids between iEgilops and wheat, have perpetuated 

 themselves under culture since 1857, icith a rapid but varying 

 increase of fertility in each generation. In the fourth generation 

 the plants, still retaining their intermediate character, had 

 become as fertile as common cultivated wheat. 



The indirect evidence in favour of the Pallasian doctrine 

 appears to me to be extremely strong. In the earlier chapters 

 I have shown that our various breeds of the dog are descended 

 from several wild species ; and this probably is the case with 

 sheep. There can be no doubt that the Zebu or humped 

 Indian ox belongs to a distinct species from European cattle : 

 the latter, moreover, are descended from two forms, which 

 may be called either species or races. We have good evidence 



28 'Act. Acad. St. Petersburg,' (1st series), p. 61. 



1780, part ii. pp. 84, 100. 30 ' Bull. Bot. Soc. de France,' Dec, 



29 ' Annales des Sc. Nat.' torn. sxi. 27th, 1861, torn. viii. p. 612. 



