^ 



100 CAUSES WHICH CHECK Chap. XVI. 



CHAPTER XVI. 



CAUSES WHICH INTEEFEKE WITH THE FREE CROSSING OF 

 VARIETIES — INFLUENCE OF DOMESTICATION ON FERTILITY. 



DIFFICULTIES IN JUDGING OF THE FERTILITY OF VARIETIES WHEN CROSSED 



HE 



AND SEXUAL PREFERENCE — VARIETIES OF WHEAT SAID TO BE STERILE WHEN 

 CROSSED — VARIETIES OF MAIZE, VERBASCUM, HOLLYHOCK, 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 in- 

 superable distinction between races and species. The import- 

 ance 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 1 has remarked that 

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

 inter se, are indefinitely fertile; yet, if a shade of infertility 

 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 recrossed in various ways, that 

 any sterility, if it had existed, would from being injurious 



i < 



Journal de Physiolog.,' torn, ii., 1859, p. 385. 



