RECAPITULATION. 449 



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 that a cross between the distinct individuals of the same 

 variety, and between distinct varieties, increases the number 

 of their offspring, and certainly gives to them increased size 

 and vigor. This is chiefly owing to the forms which are 

 crossed having been exposed to somewhat different condi- 

 tions 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 con- 

 ditions, the good derived from crossing is often much dimin- 

 ished 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 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 

 organizations. 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 en- 

 countered 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 how- 

 ever 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 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 fox' yery 



