430 PHILOSOPHY OF ZOOLOGY. 
place among individuals of nearly related species. Thus, 
among quadrupeds, the mule is the produce of the union 
of the horse and the ass. The jackall and the wolf both 
breed with the dog. Among birds, the canary and gold- 
finch breed together, the Muscovy and common duck, and 
the pheasant and hen. Among fishes, the carp has been 
known to breed with the tench, the crusian, and even the 
trout *. 
2. The Parents must be in a confined or domesticated 
state.—In all those hybridous productions which have yet 
been obtained, there is no example of individuals of one 
species giving a sexual preference to those of another. 
Among quadrupeds and birds, those individuals of different 
species which have united, have been confined, and exclud- 
ed from all intercourse with those of their own kind. In 
the case of hybridous fishes, the ponds in which they have 
been produced have been small and overstocked, and no 
natural proportion observed between the males and females 
of the different kinds. As the impregnating fluid, m such 
situations, is spread over the eggs after exclusion, a portion 
of it belonging to one species may have come in contact 
with the unimpregnated eges of another species, by the ac- 
cidental movements of the water, and not in consequence 
of any unnatural effort. 
In all cases of this unnatural union among birds or qua- 
dirupeds, a considerable degree of aversion is always exhibit- 
ed, a circumstance which never occurs among individuals 
of the same species +. 
* Phil. Trans. 1771, p. 318. 
+ Joun Hunter having succeeded in producing a breed between the 
dog and the wolf and jackall, hastily concluded that they all belonged to the 
species, by overlooking the aversion to the intercourse which was exhibited. 
Phil. Trans. 1757, p. 253. and 1789, p. 160. 
