44 SUMMAEY, [Chap. IX 



close similarity in the offspring of crossed species, and 

 of crossed varieties. If we look at species as having 

 been specially created, and at varieties as having been 

 produced by 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 

 generally, 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 species, and is eminently susceptible to the action 

 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 

 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 



