Mr. Orton relies chiefly on the evidence presented by _hybrids_, the progeny of distinct species, or by crosses between the most distinct varieties embraced within a single species, to establish his law. The examples adduced are chiefly from the former. The _mule_ is the progeny of the male ass and the mare; the _hinny_ that of the horse and the she ass. Both hybrids are the produce of the same set of animals. They differ widely, however, in their respective characters--the mule in all that relates to its external characters having the distinctive features of the ass,--the hinny, in the same respects having all the distinctive features of the horse; while in all that relates to the internal organs and vital qualities, the mule partakes of the character of the horse, and the hinny of those of the ass. Mr. Orton says--"The mule, the produce of the male ass and mare, is essentially a _modified ass_: the ears are those of an ass somewhat shortened; the mane is that of the ass, erect; the tail is that of an ass; the skin and color are those of an ass somewhat modified; the legs are slender and the hoofs high, narrow and contracted, like those of an ass. In fact, in all these respects it is an ass somewhat modified. The body and barrel, however, of the mule are round and full, in which it differs from the ass and resembles the mare.
The hinny, on the other hand, the produce of the stallion and she ass, is essentially a _modified horse_. The ears are those of a horse somewhat lengthened; the mane flowing; the tail bushy, like that of the horse; the skin is finer, like that of the horse, and the color varies also, like the horse; the legs are stronger and the hoofs broad and expanded like those of the horse. In fact, in all these respects it is a horse somewhat modified. The body and barrel, however, of the hinny are flat and narrow, in which it differs from the horse and resembles the she ass.
A very curious circumstance pertains to the voice of the mule and the hinny. The mule _brays_, the hinny _neighs_. The why and wherefore of this is a perfect mystery until we come to apply the knowledge afforded us by the law before given. The male gives the locomotive organs, and the muscles are amongst these; the muscles are the organs which modulate the voice of the animal; the mule has the muscular structure of its sire, and brays; the hinny has the muscular structure of its sire, and neighs."