RECAPITULATION AND CONCLUSION 501 



animals to breed freely under diversified circumstances ; and 

 this again apparently follows from their having been grad- 

 ually 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 distinct individuals of the same 

 variety, and between distinct varieties, 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 

 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 ex- 

 posed 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, ov.-ing 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 ele- 

 phant 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 domesti- 



