DURATION OF SPECIES. 3i7 



mite portions of them, should reenforce vitality, we 

 do not know, and can hardly conjecture. But this 

 must be the meaning of sexual reproduction. 



The conclusion of the matter, from the scientific 

 point of view, is, that sexually-propagated varieties or 

 races, although liable to disappear through change, 

 need not be expected to wear out, and there is no proof 

 that they do ; but, that non-sexually propagated va- 

 rieties, though not especially liable to change, may 

 theoretically be expected to wear out, but to be a very 

 long time about it. 



II. 



Do Sjiecies wear out f and if not, why not f 



The question we have just been considering was 

 merely whether races are, or may be, as enduring as 

 species. As to the inherently unlimited existence of 

 species themselves, or the contrary, this, as we have 

 said, is a geological and very speculative problem. Not 

 a few geologists and naturalists, however, have con- 

 cluded, or taken for granted, that species have a natu- 

 ral term of existence — that they culminate, decline, 

 and disappear through exhaustion of specific vitality, 

 or some equivalent internal cause. As might be ex- 

 pected from the nature of the inquiry, the facts which 

 bear upon the question are far from decisive. If the 

 fact that species in general have not been interminable, 

 but that one after another in long succession has be- 

 come extinct, would seem to warrant this conclusion, 

 the persistence through immense periods of no incon- 



