Chap. XIX. HYBRIDISM. 191 



with undiminished fertility repeated changes of conditions 

 might be expected to produce varieties, which would be little 

 liable to have their reproductive powers injuriously affected by 

 the act of crossing with other varieties which had originated in 

 a like manner. 



Certain naturalists have recently laid too great stress, as it 

 appears to me, on the difference in fertility between varieties 

 and species when crossed. Some allied species of trees cannot 

 be grafted on each other,— all varieties can be so grafted. Some 

 allied animals are affected in a very different manner by the 

 same poison, but with varieties no such case until recently was 

 known, but now it has been proved that immunity from certain 

 poisons stands in some cases in correlation with the colour of 

 the hair. The period of gestation generally differs much with 

 distinct species, but with varieties until lately no such difference 

 had been observed. The time required for the germination 

 of seeds differs in an analogous manner, and I am not aware 

 that any difference in this respect has as yet been detected 

 with varieties. Here we have various physiological differences, 

 and no doubt others could be added, between one species and 

 another of the same genus, which do not occur, or occur with 

 extreme rarity, in the case of varieties; and these differences 

 are apparently wholly or in chief part incidental on other con- 

 stitutional differences, just in the same manner as the sterility 

 of crossed species is incidental on differences confined to the 

 sexual system. Why, then, should these latter differences, how- 

 ever serviceable they may indirectly be in keeping the inha- 

 bitants of the same country distinct, be thought of such para- 

 mount importance, in comparison with other incidental and 

 functional differences? No sufficient answer to this question 

 can be given. Hence the fact that the most distinct domestic 

 varieties are, with rare exceptions, perfectly fertile when crossed, 

 and produce fertile offspring, whilst closely allied species are, 

 with rare exceptions, more or less sterile, is not nearly so 

 formidable an objection as it appears at first to the theory of 

 the common descent of allied species. 



