432 PHILOSOPHY OF ZOOLOGY. 
of procreation, and giving birth to a fertile progeny, we 
may with confidence conclude that the individuals so acting 
belong to one species. When, on the contrary, we observe 
individuals in a domesticated or confined state, and without 
the liberty of choice, engaging reluctantly in the gratifica- 
tion of the procreative instinct, and giving birth to a barren 
progeny, we may be certain that such belong to different 
species, and that the union has been unnatural. This cir- 
cumstance of procreating willingly, and producing a fertile 
progeny, is the only infallible criterion of species,—the 
basis of all the methods of arrangement in zoology, and all 
the truths of anatomy and physiology. 
END OF VOLUME FIRST. 
