n THE ORIGIN OF SPECIES 47 



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 con- 

 clusions are summed up as follows, 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 

 different and sometimes widely different, in reciprocal crosses 



