170 HYBRIDISM. Chap. XIX. 



of species or by single species, whicli wlien brought together 

 and crossed are found to be more or less sterile ; now it could 

 clearly have been no advantage to such separated species to 

 have been rendered mutually sterile, and consequently this 

 could not have been effected through natural selection ; but 

 it may perhaps be argued, that, if a species were rendered 

 sterile with some one compatriot, sterility with other species 

 Wwuld follow as a necessary consequence. In the second 

 place, it is as much opposed to the theory of natural selection, 

 as to the theory of special creation, that in reciprocal crosses 

 the male element of one form should have been rendered 

 utterly impotent on a second form, whilst at the same time 

 the male element of this second form is enabled freely to 

 fertilise the first form ; for this peculiar state of the repro- 

 ductive system could not possibly have been advantageous 

 to either species. 



In considering the probability of natural selection having 

 come into action in rendering species mutually sterile, one 

 of the greatest difficulties will be found to lie in the existence 

 of many graduated steps from slightlj^ lessened fertility to 

 absolute sterility. It may be admitted, on the principle 

 above explained, that it would profit an incipient species if it 

 were rendered in some slight degree sterile when crossed with 

 its parent-form or with some other variety ; for thus fewer 

 bastardised and deteriorated offspring would be produced to 

 commingle their blood with the new species in process of 

 formation. But he who will take the trouble to reflect on the 

 steps by which this first degree of sterility could be increased 

 through natural selection to that higher degree which is 

 common to so many species, and which is universal with 

 species which have been differentiated to a generic or family 

 rank, will find the subject extraordinarily complex. After 

 mature reflection it seems to me that this could not have been 

 effected through natural selection. Take the case of any two 

 species which, when crossed, produce few and sterile offspring ; 

 now, what is there which could favour the survival of those 

 individuals which happened to be endowed in a slightly 

 higher degree with mutual infertility, and which thus 

 approached by one small step towards absolute sterility "^ 



