78 CAUSES WIIICH CHECK Chap. XVI. 



CHAPTER XVI. 



CAUSES WHICH INTERFERE WITH THE FREE CROSSING OF 

 VARIETIES — INFLUENCE OF DOMESTICATION ON FERTILITY. 



DIFFICULTIES IN JUDGING OF THE FERTILITY OF VARIETIES WHEN" CROSSED 

 — VARIOUS CAUSES WHICH KEEP VARIETIES DISTINCT, AS THE PERIOD 

 OF BREEDING AND SEXUAL PREFERENCE — VARIETIES OF WHEAT SAID TO 

 BE STERILE WHEN CROSSED VARD2TD3S OF MAIZE, YERBASCUM, HOLLY- 

 HOCK, GOURDS, MELONS, AND TOBACCO, RENDERED IN SOME DEGREE 

 MUTUALLY STERILE — DOMESTICATION ELIMINATES THE TENDENCY TO 



STERILITY NATURAL TO SPECIES WHEN CROSSED ON THE INCREASED 



FERTILITY OF UNCROSSED ANIMALS AND PLANTS FROM DOMESTICATION 

 AND CULTIVATION. 



The domesticated races of both animals and plants, when 

 crossed, are, with extremely few exceptions, quite prolific, — in 

 some cases even more so than the purely-bred parent-races. 

 The offspring, also, raised from such crosses are likewise, as 

 we shall see in the following chapter, generally more vigorous 

 and fertile than their parents. On the other hand, species 

 when crossed, and their hybrid offspring, are almost invariably 

 in some degree sterile ; and here there seems to exist a broad 

 and insuperable distinction between races and species. The 

 importance of this subject as bearing on the origin of species 

 is obvious ; and we shall hereafter recur to it. 



It is unfortunate how few precise observations have been 

 made on the fertility of mongrel animals and plants during 

 several successive generations. Dr. Broca x has remarked 

 that no one has observed whether, for instance, mongrel dogs, 

 bred inter se, are indefinitely fertile ; yet, if a shade of in- 

 fertility be detected by careful observation in the offspring of 

 natural forms when crossed, it is thought that their specific 

 distinction is proved. But so many breeds of sheep, cattle, 

 pigs, dogs, and poultry, have been-crossed and reerossed in 

 various ways, that any sterility, if it had existed, would from 

 beino- injurious almost certainly have been observed. In 



1 ' Journal ie Physiolog.,' torn, ii., 1859, p. 385. 



