m THE ORIGIN OF SPECIES AT 
‘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 
-erossed; 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 
