of the minute scales which cover their wings, etc.  Among Crustacea, the Genera of Shrimps vary in the form of the claws, in the structure of the parts of the mouth, in the articulations of their feelers, etc.  Among Worms, the different Genera of the Leech Family are combined upon the form of the disks by which they attach themselves, upon the number and arrangement of their eyes, upon the structure of the hard parts with which the mouth is armed, etc.  Among Cephalopods, the Family of Squids contains several Genera distinguished by the structure of the solid shield within the skin of the back, by the form and connection of their fins, by the structure of the suckers with which their arms are provided, by the form of their beak, etc.  In every Class, we find throughout the Animal Kingdom that there is no sound basis for the discrimination of Genera except the details of their structure; but in order to define them accurately an extensive comparison of them is indispensable, and in characterizing them only such features should be enumerated as are truly generic; whereas in the present superficial method of describing them, features are frequently introduced which belong not only to the whole Family, but even to the whole Class which includes them.

X.

There remains but one more division of the Animal Kingdom for our consideration, the most limited of all in its circumscription,—­that of Species.  It is with the study of this kind of group that naturalists generally begin their investigations.  I believe, however, that the study of Species as the basis of a scientific education is a great mistake.  It leads us to overrate the value of Species, and to believe that they exist in Nature in some different sense from other groups; as if there were something more real and tangible in Species than in Genera, Families, Orders, Classes, or Branches.  The truth is, that to study a vast number of Species without tracing the principles that combine them under more comprehensive groups is only to burden the mind with disconnected facts, and more may be learned by a faithful and careful comparison of a few Species than by a more cursory examination of a greater number.  When one considers the immense number of Species already known, naturalists might well despair of becoming acquainted with them all, were they not constructed on a few fundamental patterns, so that the study of one Species teaches us a great deal for all the rest.  De Candolle, who was at the same time a great botanist and a great teacher, told me once that he could undertake to illustrate the fundamental principles of his science with the aid of a dozen plants judiciously selected, and that it was his unvarying practice to induce students to make a thorough study of a few minor groups of plants, in all their relations to one another, rather than to attempt to gain a superficial acquaintance with a large number of species.  The powerful influence