274 ^aj Sjerntcrns, (Stesmgs, atifo 



with the pollen of another species than with their own ; 

 and there are others, such as certain fuel, whose male 

 element will fertilize the ovule of a plant of distinct spe- 

 cies, while the males of the latter species are ineffective 

 with the females of the first. So that, in the last-named 

 instance, a physiologist, who should cross the two species 

 in one way, would decide that they were true species ; 

 while another, who should cross them in the reverse 

 way, would, with equal justice, according to the rule, 

 pronounce them to be mere races. Several plants, which 

 there is great reason to believe are mere varieties, are 

 almost sterile when crossed ; while both animals and 

 plants, which have always been regarded by naturalists 

 as of distinct species, turn out, when the test is applied, 

 to be perfectly fertile. Again, the sterility or fertility 

 of crosses seems to bear no relation to the structural 

 resemblances or differences of the members of any two 

 groups. 



Mr. Darwin has discussed this question with singular 

 ability and circumspection, and his conclusions are 

 summed up as follow, at page 276 of his work : 



" First crosses between forms sufficiently distinct to be ranked as 

 species, and their hybrids, are very generally, but not universally, 

 sterile. The sterility is of all degrees, and is often so slight that the 

 two most careful experimentalists who have ever lived have come to 

 diametrically opposite conclusions in ranking forms by this test. The 

 sterility is innately variable in individuals of the same species, and is 

 eminently susceptible of favourable and unfavourable conditions. The 

 degree of sterility does not strictly follow systematic affinity, but is 

 .governed by several curious and complex laws. It is generally dif- 

 ferent, and sometimes widely different, in reciprocal crosses between 

 the same two species. It is not always equal in degree in a first cross, 

 and in the hybrid produced from this cross. 



"In the same manner as in grafting trees, the capacity of one 

 species or variety to take on another is incidental on generally 

 unknown differences in their ^vegetative systems ; so in crossing, the 

 greater or lesy facility of one species to unite with another is inci- 

 dental on unknown differences in their reproductive systems. There 



