276 HYBKIDISM. Chap. VIII. 



astonishing fact. But it harmonises perfectly with the 

 view that there is no essential distinction between spe- 

 cies and varieties. 



Summary of Chapter. — 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 

 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 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 

 them crossing and blending in nature, than to tliink 

 that trees have been specially endowed with various and 

 somewhat analogous degrees of difficulty in being grafted 

 together in order to prevent them becoming inarched 

 in our forests. 



The sterility of first crosses between pure species, 

 which have their reproductive systems perfect, seems 



