330 ORIGIN OF SPECIES 



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 simi- 

 larity in the offspring of crossed species, and of crossed vari- 

 eties. 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 har- 

 monises 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 indi- 

 viduals 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 vege- 

 tative 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 pre- 

 vent their inarching in our forests. 



