The Art of Love by W. F. Robie, M.D.

The Kama Sutra Chapter I

The horse and mare, the bull and deer, form the high union, while the horse and deer form the highest union. On the female side, the elephant and bull, the mare and hare, form low unions, while the elephant and the hare make the lowest unions.

There are, then, nine kinds of union according to dimensions. Amongst all these, equal unions are the best, those of a superlative degree, the highest and the lowest, are the worst, and the rest are middling, and with them the high are better than the low. (High unions are said to be better than low ones, for in the former it is possible for the male to satisfy his own passions without injuring the female, while in the latter it is difficult for the female to be satisfied by any means.)