406 RECAPITULATION. [Chap. XV. 



increased size and vigour. This is chiefly owing to the forms which 

 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 

 conditions, 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 arc 

 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 then- 

 conditions of life, from being compounded of two distinct organisa- 

 tions. 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, whilst two domesticated varieties when crossed 

 and their mongrel offspring are perfectly fertile. 



Turning to geographical distribution, the difficulties encountered 

 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 common 

 parents ; and therefore, in however distant and isolated parts of the 

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

 gensrations have travelled from some one point to all 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 



