CHAPTER IV.

WE have yet to consider another class of phenomena, those relating to the 
production of hybrids, which have been regarded in a very different light with 
reference to their bearing on the question of the permanent distinctness of 
species ; some naturalists considering them as affording the strongest of all 
proofs in favour of the reality of species ; others, on the contrary, appealing 
to them as countenancing the opposite doctrine, that all the varieties of 
organization and instinct now exhibited in the animal and vegetable kingdoms, 
may have been propagated from a small number of original types.
 

In regard to the mammifers and birds, it is found that no sexual union will take 
place between races which are remote from each other in their habits and 
organization ; and it is only in species that are very nearly allied that such 
unions produce offspring. It may be laid down as a general rule, admitting of 
very few exceptions among quadrupeds, that the hybrid progeny is steril, and 
there seem to be no well-authenticated examples of the continuance of the mule 
race beyond one generation. The principal number of observations and experiments 
relate to the mixed offspring of the horse and the ass ; and in this case it is 
well established, that the male-mule can generate and the female-mule produce. 
Such cases occur in Spain and