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 —is 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 Kélreuter. To give an instance: Mirabilis 
jalapa can easily be fertilized by the pollen of MZ. longifora, 
and the hybrids thus produced are sufficiently fertile ; -but 
Kélreuter tried more than two hundred times, during eight 
