RECAPITULATION 279 



posed 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 de- 

 rived 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 condi- 

 tions, 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 domes- 

 ticated 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 ren- 

 dered 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 conditions of life, from being 

 compounded of two distinct organizations. He who will 

 explain in a definite manner why, for instance, an ele- 

 phant or a fox will not breed under confinement in its 

 native country, while 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 ques- 

 tion why two distinct species, when crossed, as well as 

 their hybrid off'spring, 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 



encountered on the theory of descent with modification 



— SriExcE — 29 



