There are, however, many cases in which we cannot distinguish these characters, nor pronounce on their differences with certainty: there are others in which we are obliged to suspend our opinions, and in a great number of others we have not the smallest ray of light for our guide; for, independent of the uncertainty arising from the contrariety of assertions respecting recorded facts, independent of the doubts resulting from the inaccuracy of those who have endeavoured to observe Nature, the greatest obstacle to the advancement of knowledge, is our ignorance of a great number of effects which time has not disclosed to us, and which can only be revealed to posterity by experience, and the most accurate observations; in the mean time we stray in darkness, perplexed between prejudices and probabilities, ignorant even of possibilities, and every moment confounding the opinions of men with the acts of Nature. Examples are in abundance; but, without quitting our subject, we know that the goat and the sheep couple together; though we are still to learn whether the mule from this commixture is sterile or fruitful. We are apt to conclude that mules in general, are barren, because those produced from the he-ass and mare, or the horse and she-ass, are sterile. But this opinion may have no foundation, since the ancients positively assert, that the mule produces at seven years old and that it can produce with the mare; they say also that the she-mule is capable of conception, but that she cannot bring her fruit to perfection. It is necessary therefore, to destroy or confirm the truth of these facts, since they obscure the real distinction of animals and the theory of their generation; and though we know distinctly the species of all the animals which surround us, yet we are ignorant what might be produced by an intermixture among themselves, or with foreign animals. We are but ill informed of the jumar, an animal said to be the produce of a cow and an ass, or a mare and a bull. We are also ignorant whether the zebra would not produce with the horse or the ass, or the broad-tailed Barbary ram with a common ewe; whether the chamois goat be any thing more than a common goat in a wild state, or whether an intermixture would not form an intermediate race; whether the monkeys are of different species, or, like that of the dog, it is one and the same, but varied by a great number of different breeds; whether the dog can produce with the fox and the wolf, the stag with the cow, &c. Our ignorance in most of these cases is almost invincible, and the experiments which would decide them require more time, care, and expence, than the life and fortune of most men can permit.