238 LAY SERMONS, ESSAYS, AND REVIEWS. [xn. 



working of other pollen, are obstacles of no less magnitude in 

 applying the test to them. And in both animals and plants is 

 superadded the further difficulty, that experiments must be con 

 tinued over a long time for the purpose of ascertaining the 

 fertility of the mongrel or hybrid progeny, as well as of the first 

 crosses from which they spring. 



Not only do these great practical difficulties lie in the way of 

 applying the hybridization test, but even when this oracle can 

 be questioned, its replies are sometimes as doubtful as those of 

 Delphi. For example, cases are cited by Mr. Darwin, of plants 

 which are more fertile with the pollen of another species than with 

 their own ; and there are others, such as certain fuci, whose male 

 element will fertilize the ovule of a plant of distinct species, 

 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 

 follows, at page 276 of his work : 



&quot; 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 favour 

 able 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 



