4:4: SUMMARY. [Chap. IX 



pfoducetf' "Sy-secondary laws^ this similarity would be 

 an astonishing fact. But it harmonises perfectly with 

 the view that there is no essential distinction between 

 species and varieties. 



Summary of Chapter. 



First crosses between forms, sufficiently distinct to 

 be ranked as species, and their hybrids, are very gen- 

 erally, but not universally, sterile. The sterility is of 

 all degrees, and is often so slight that the most careful 

 experimentalists have arrived at diametrically opposite 

 conclusions in ranking forms by this test. The sterility 

 is innately variable in individuals of the same spe- 

 cies, and is eminently susceptible to the action of fa- 

 vourable 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 re- 

 ciprocal crosses between 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 capac- 

 ity in one species or variety to take on another, is inci- 

 dental on differences, generally of an unknown nature, 

 in their vegetative sjfstems, so in crossing, the greater 

 or less facility of one species to unite with another is in- 

 cidental on unknown differences in their reproductive 

 systems. There is no more reason to think that species 

 have been specially endowed with various degrees of 

 steriHty to prevent their crossing and blending in na- 

 ture, than to think that trees have been specially en- 

 dowed with various and somewhat analogous degrees of 



