RECAPITULATION AND CONCLUSION 415 



are crossed having been exposed to somewhat different conditions 

 of life; for I have ascertained by a laborious series of experiments 

 that if all the individuals of the same variety be subjected during 

 several generations to the same conditions, the good derived from 

 crossing is often much diminished or wholly disappears. This is 

 one side of the case. On the other side, we know that species which 

 have long been exposed to nearly uniform conditions, when they 

 are subjected under confinement to new and greatly changed con- 

 ditions, either perish, or if they survive, are rendered sterile, 

 though retaining perfect health. This does not occur, or only in a 

 very slight degree, with our domesticated productions, which 

 have long been exposed to fluctuating conditions. Hence when we 

 find that hybrids produced by a cross between two distinct species 

 are few in number, owing to their perishing soon after conception 

 or at a very early age, or if surviving that they are rendered more 

 or less sterile, it seems highly probable that this result is due to 

 their having been in fact subjected to a great change in their con- 

 ditions of life, from being compounded of two distinct organiza- 

 tions4 He who will explain in a definite manner why, for instance, 

 an elephant or a fox will not breed under confinement in its native 

 country, whilst the domestic pig or dog will breed freely under the 

 most diversified conditions, will at the same time be able to give a 

 definite answer to the question why two distinct species, when 

 crossed, as well as their hybrid offspring, are generally rendered 

 more or less sterile, while two domesticated varieties when crossed 

 and their mongrel offspring are perfectly fertile.' 



Turning to geographical distribution, the difficulties encoun- 

 tered on the theory of descent with modification are serious 

 enough. All the individuals of the same species, and all the species 

 of the same genus, or even higher group, are descended from com- 

 mon parents ; and therefore, in however distant and isolated parts 

 of the world they may now be found, they must in the course of 

 successive generations have travelled from some one point to aU 

 the others. We are often wholly unable even to conjecture how 

 this could have been effected. Yet, as we have reason to believe 

 that some species have retained the same specific form for very 

 long periods of time, immensely long as measured by years, too 

 much stress ought not to be laid on the occasional wide diffusion 

 of the same species ; for during very long periods there will always 

 have been a good chance for wide migration by many means. A 

 broken or interrupted range may often be accounted for by the 

 extinction of the species in the intermediate regions. It cannot be 

 denied that we are as yet very ignorant as to the full extent of 



