208 HABIT AND INTELLIGENCE, [chap. 



has been a very generally received oi^inion that this cri- 

 terion of distinction between species is absolute ; that the 

 distinction of species thus made and thus marked is a per- 

 fectly impassable barrier ; and that, consequently, not only 

 the first origin of life, but also the origin of every distinct 

 species, needed a distinct act of Creative Power. In my 

 opinion, however, Darwin has fully answered this objection 

 by the great mass of facts brought together on this subject 

 in the chapter on Hybridism, in his great work on the 

 " Origin of Species." As that work is very well known 

 and very readable, I will only state his conclusions in the 

 barest outline ; some of them have been stated already in 

 the last chapter, but I will repeat them here. 



It is true that similar organisms produce offspring 

 together, and that unlike organisms will not produce 

 not abso- offspring together. But it is not true that this distinction 

 ^^^^' is rigid and absolute ; on the contrary, it admits of grada- 



tion. Between perfect mutual fertility and perfect mutual 

 sterility there are an indefinite number of gradations. 

 Sometimes forms that are regarded as distinct species are 

 perfectly fertile with each other, taking the production of 

 the average number of seeds as the test of perfect fertility ; 

 sometimes forms that are regarded as only varieties of the 

 same species are not perfectly fertile with each other. The 

 fertility of different varieties and species with each other 

 admits of various degrees, as estimated by the number of 

 seeds produced. Sometimes hybrids are perfectly normal 

 organisms ; sometimes they are healthy, but infertile ; 

 sometimes they are weak, and perish early. 

 Eeasou of It is important to observe that we do not know on what 

 ^eH^fty the mutual sterility of distinct species depends. It is cer- 

 uuknown. tainly not on mere visible unlikeness : the various domestic 

 breeds of pigeons are all perfectly fertile together, and yet 

 they are more unlike in external appearance than many 

 animals are to each other which are universally classed as 

 distinct species, and most probably are not fertile together. ^ 



1 I say viost probably, because the fact is very diflBcult to ascertain, as 

 few animals breed freely in confinement. Tbe case of the mule between 

 the horse aud the ass, however, is an important one on this subject ; 



