478 RECAPITULATION. 



domestic animals to breed freely under diversifiea circum- 

 stances; and this again apparently follows from their 

 having been gradually accustomed to frequent changes in 

 their conditions of life. 



A double and parallel series of facts seems to throw much 

 light on the sterility of species, when first crossed, and of 

 their hybrid offspring. On the one side, there is good 

 reason to believe that slight changes in the conditions of 

 life give vigor and fertility to all organic beings. We know 

 also 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 in- 

 creased size and vigor. This is chiefly owing to the forms 

 which are crossed having been exposed to somewhat dif- 

 ferent conditions of life ; for I have ascertained by a labori- 

 ous series of experiments that if all the individuals of the 

 same variety be subjected during several generations to the 

 came 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 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 gener- 

 ally rendered more or less sterile, while two domesticated 

 varieties when crossed and their mongrel offspring are per- 

 fectly fertile. 



