262 SUMMARY. [Chap. IX. 



having been produced by secondary laws, this similarity would he 

 an astonishing fact. But it harmonises perfectly with the view 

 that there is no essential distinction between specios 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 of one 

 species or variety to take on another, is incidental on • inferences, 

 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 various and somewhat analogous degrees 

 of difficulty in being grafted together in order to prevent their 

 inarching in our forests. 



The sterility of first crosses and of their hybrid progeny has not 

 been acquired through natural selection. In the case of first crosses 

 it seems to depend on several circumstances; in some instances in 

 chief part on the early death of the embryo. In the case of hybrids, 

 it apparently depends on their whole organisation having been 

 disturbed by being compounded from two distinct forms; the 

 sterility being closely allied to that which so frequently affects pure 

 species, when exposed to new and unnatural conditions of life. 

 He who will explain these latter cases will be able to explain the 

 sterility of hybrids. This view is strongly supported by a parallelism 

 of another kind : namely, that, firstly, slight changes in the con- 

 ditions of life add to the vigour and fertility of all organic beings ; 

 and secondly, that the crossing of forms, which have been exposed to 

 slightly different conditions of life or which have varied, favours the 

 size, vigour, and fertility of their offspring. The facts given on tho 



