CHAP. IX.] SUMMARY. 245 



monstrous in character, than with hybrids, which are descended 

 from species slowly and naturally produced. On the whole, I 

 entirely agree with Dr. Prosper Lucas, who, after arranging an 

 enormous body of facts with respect to animals, comes to the con- 

 clusion that the laws of resemblance of the child to its parents are 

 the same, whether the two parents differ little or much from each 

 other, namely, in the union of individuals of the same variety, or 

 of different varieties, or of distinct species. 



Independently of the question of fertility and sterility, in all 

 other respects there seems to be a general and 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 com- 

 plex 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 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 



