270 RECAPITULATION. [Chap. XV. 



for I have ascertained by a laborious series of experi- 

 ments 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 condi- 

 tions, 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 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 

 conditions of life, from being compounded of two distinct 

 organisations. 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, 

 arc generally rendered more or less sterile, whilst two 

 dom I 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 



