50 THE ORIGIN OF SPECIES 



produced by secondary laws, this similarity would be an 

 astonishing fact. But it harmonizes perfectl}^ with the 

 view that there is no essential distinction between species 

 and varieties. 



Summary 



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 de- 

 grees, and is often so slight that the most careful ex- 

 perimentalists have arrived at diametrically opposite con- 

 clusions in ranking forms by this test. The sterility is 

 innately variable in individuals of the same species, and 

 is eminently susceptible to the action of favorable and 

 unfavorable conditions. The degree of sterility does not 

 strictly follow systematic affinity, but is governed by sev- 

 eral curious and complex laws. It is generally different, 

 and sometimes widely different in reciprocal crosses be- 

 tween the same two species. It is not always equal in 

 degree in a first cross and in the hybrids produced from 

 this cross. 



In the same manner as in grafting trees, the capacity 

 in one species or variety to take on another is incidental 

 on differences, generally of an unknown nature, in their 

 vegetative systems; so, in crossing, the greater or less 

 facility of one species to unite with another is incidental 

 on unknown differences in their reproductive systems. 

 There is no more reason to think that species have 

 been specially endowed with various degrees of sterility 

 to prevent their crossing and blending in nature, than to 

 think that trees have been specially endowed with vari- 

 ous and somewhat analogous degrees of difficolty in 



