CuAr. VIII. OF FIRST CROSSES AND OF IIYBKIDS. 247 



come into action, in rendering species mutually sterile, one 

 frrcat clifTiculty will he found to lie in the existence of many 

 graduated steps from slin^litly-lesscned fertility to absolute 

 sterility. It may be admitted, on the principle above ex- 

 plained, that it would \n-oi\t an incipient species if it were 

 rendered in some slight degree sterile when crossed with its 

 j)ar(>nt-form or with some other variety ; for thus fewer bas- 

 tartUzed and deteriorated odspiing would be produced to com- 

 mingle their blood with the new species in jirocess of forma- 

 tion. But he who will take the trouble to rellcct on the steps 

 by which this first degree of steriUty could be increased through 

 natural selection to that high degree which is common with so 

 many species, and which is iniiversol with species which have 

 been diflerentiated to a generic or family rank, will find the 

 subject extraordinarily complex. After mature rellection it 

 seems to me that this could not have been effected through 

 natural selection ; for it could have been of no direct advantiigc 

 to an individual animal to breed badly with another indi\-idual 

 of a dilVcrent variety, and thus to leave few offspring ; conse- 

 quently such individuals could not have been preserved or se- 

 lected. Or take the case of two species which, in their present 

 state when crossed, produce few and sterile oflspring ; now, 

 what is there which coidd favor the survival of those individuals 

 which ha})pe!ied to be endowed in slightly-higher degree Avith 

 nnitual infertility, and which thus approached by one small step 

 toward absolute sterility ? Yet an advance of this kind, if the 

 theory of natural selection be brought to bear, must have in- 

 cessantly occurred with many species, for a multitude are mutu- 

 ally quite barren. With sterile neuter insects we have reason 

 to believe that modifications in their structure and fertility have 

 been slowly accumulated by natural selection, from an advan- 

 tage having been thus indirectly given to the conununity to 

 which they belonged over other communities of the same spe- 

 cies ; but an individual animal not belonging to a social com- 

 numity, if rendered slightly sterile when crossed with some 

 other variety, would not thus itself gain any advantage or in- 

 directly give any advantage to the other individnals of the 

 same variety, thus leading to their preservation. From these 

 considerations I infer, as far as animals are concerncil, that the 

 various degrees of lessened fertility which occur with species 

 when crossed cannot have been slowly accumulated by means 

 of natural s(>l(>ction. 



With plants, it is possible that the case may be somewhat 



