CHAP. XV.] RECAPITULATION. 381 



when crossed. This elimination of sterility apparently follows 

 from the same cause which allows our domestic animals to breed 

 freely under diversified circumstances ; 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 vigour and fertility to 

 all organic beings. We know also that a cross between the dis- 

 tinct individuals of the same variety, and between distinct varie- 

 ties, increases the number of their offspring, and certainly gives to 

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

 which are crossed having been exposed to somewhat different con- 

 ditions of life ; for I have ascertained by a laborious series of 

 experiments that if all the individuals of the same variety be sub- 

 jected during several generations to the same conditions, the good 

 derived from crossing is often much diminished OF wholly disap- 

 pears. This is one side of the case. On the other side, we know 

 that species which have long been exposed to nearly uniform con- 

 ditions, 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 pro- 

 ductions, 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, are generally rendered more or less sterile, whilst 

 two domesticated varieties when crossed and their mongrel off- 

 spring 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 sue- 



