That we may understand the true meaning of these divisions, we must not be misled by the name given by naturalists to this kind of group. Here, as in so many other instances, a word already familiar, and that had become, as it were, identified with the special sense in which it had been used, has been adopted by science and has received a new signification. When naturalists speak of Families among animals, they do not allude to the progeny of a known stock, as we designate, in common parlance, the children or the descendants of known parents by the word family; they understand by Families natural groups of different kinds of animals, having no genetic relations so far as we know, but agreeing with one another closely enough to leave the impression of a more or less remote common parentage. The difficulty here consists in determining the natural limits of such groups, and in tracing the characteristic features by which they may be defined; for individual investigators differ greatly as to the degree of resemblance existing between the members of many Families, and there is no kind of group which presents greater diversity of circumscription in the classifications of animals proposed by different naturalists than these so-called Families. It should be remembered, however, that, unless a sound criterion be applied to the limitation of Families, they, like all other groups introduced into zoölogical systems, must forever remain arbitrary divisions, as they have been hitherto. A retrospective glance at the progress of our science during the past century, in this connection, may perhaps help us to solve the difficulty. Linnaeus, in his System of Nature, does not admit Families; he has only four kinds of groups,--Classes, Orders, Genera, and Species. It was among plants that naturalists first perceived those general traits of resemblance which exist everywhere among the members of natural families, and added this kind of group to the framework of their system. In France, particularly, this method was pursued with success; and the improvements thus introduced by the French botanists were so great, and rendered their classification so superior to that of Linnaeus, that the botanical systems in which Families were introduced were called natural systems, in contradistinction especially to the botanical classification of Linnaeus, which was founded upon the organs of reproduction, and which received thenceforth the name of the sexual system of plants. The same method so successfully used by botanists was soon introduced into Zoölogy by the French naturalists of the beginning of this century,--Lamarck, Latreille, and Cuvier. But, to this day, the limitation of Families among animals has not yet reached the precision which it has among plants, and I see no other reason for the difference than the absence of a leading principle to guide us in Zoölogy.