BOOK OF NATURE LAID OPEN. 57 



It is unnecessary for us to enter deeper into this 

 system of classification than merely to point out the 

 distinguishing circumstance that they have laid hold 

 on to mark the boundaries between what they call 

 a species and a variety; and this will be the easiest 

 done by choosing a familiar example for illustra- 

 tion. The horse and the ass are both of the same 

 genus of quadrupeds; and they resemble each other 

 so much, that they might well be mistaken for be- 

 ings of the same species ; but although it be found 

 that it is possible to make them procreate, yet it is 

 also known that the mule, which is produced from 

 this intermixture, is not capable of continuing its kind, 

 which circumstance induces naturalists to rank them 

 as a different species. When animals procreate to- 

 gether, and produce an offspring that is capable of 

 continuing their kind, however different they may 

 be in their appearance and other particulars, they 

 are accounted as only varieties of the same species; 

 nor is there any criterion that we know of, for dis- 

 tinguishing a variety of any particular kind of ani- 

 mal from a distinct species of the same genus, but 

 that of producing a fertile or an unfertile progeny. 

 In this particular, we believe, all naturalists are 

 agreed. 



That there are many varieties of most species of 

 animals is well known ; and as many of these va- 

 rieties, especially among domesticated animals, rea- 

 dily intercopulate with each other, and produce a 

 mixed race, participating of the qualities of both 

 parents, it necessarily must happen, that in cases 



