Darwin's Artificial and Natural Selection 149 



In regard to animals, Darwin concludes that " if the genera 

 of animals are as distinct from each other as are the genera 

 of plants, then we may infer that animals more widely distinct 

 in the scale of nature can be crossed more easily than in the 

 case of plants ; but the hybrids themselves are, I think, more 

 sterile." 



The most significant fact in this connection is that the 

 more widely different two species are, so that they are placed 

 in different families, so much the less probable is it that 

 cross-fertilization will produce any result. From this condi- 

 tion of infertility there may be traced a gradation between 

 less different forms of the same genus to almost complete, 

 or even complete, fertility between closely similar species. 

 Darwin further points out that : " The hybrids raised from 

 two species which are very difficult to cross, and which rarely 

 produce any offspring, are generally very sterile; but the 

 parallelism between the difficulty of making a first cross, and 

 the sterility of the hybrids thus produced — two classes of 

 facts which are generally confounded together — as by no 

 means strict. There are many cases, in which two pure 

 species, as in the genus Verbascum, can be united with 

 unusual facility, and produce numerous hybrid offspring, 

 yet these hybrids are remarkably sterile. On the other 

 hand, there are species which can be crossed very rarely, 

 or with extreme difficulty, but the hybrids, when at last 

 produced, are very fertile. Even within the limits of the 

 same genus, for instance in Dianthus, these two opposite 

 cases occur." 



In regard to reciprocal crosses Darwin makes the following 

 important statements : " The diversity of the result in re- 

 ciprocal crosses between the same two species was long ago 

 observed by Kolreuter. To give an instance: Mirabilis 

 jalapa can easily be fertilized by the pollen of M. longiflora, 

 and the hybrids thus produced are sufficiently fertile ; but 

 Kolreuter tried more than two hundred times, during eight 



