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. 



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