Plaints and animals are classed into families, orders, classes, and sub-kingdoms, according as they agree in a smaller or greater number of characters, and take rank in compositeness or complexity of organization. The first difficulty of naturalists is to attain to a natural mode of classification, for there ever is a tendency to proceed upon certain formal, artificial ideas of our own, as presuming that the Divine Author of nature has acted upon some plan involving merely human ideas of regularity. Other difficulties arise when we inquire by what properties various organisms should be classed, many of these being identical or parallel in plants or animals, which in other respects are very different. It cannot be said that, in either kingdom, a satisfactorily natural system has been attained, nor, I venture to say, will any such system ever be formed, until there shall be a general consent to take a natural origin of organisms as partly our guide.

It is in the meantime seen with sufficient clearness, that the families of the vegetable and animal kingdoms can be arranged in grades from the highest forms in both instances to certain humble and simple types, amongst which it is hardly possible to discern any difference. In other words, both kingdoms start from certain humble and almost common forms, and advance to types of high organization in their respective departments.