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186 which must be taken as a guide in any attempt to frame a natural classification of that group, but as having an important bearing upon some of those higher questions relating to the origin and value of differential characters generally, which have recently been brought prominently under consideration. In so doing, it is my desire to confine myself purely to the scientific and practical aspect of these questions; seeking in the first place to determine, on the legitimate basis of induction, what general principles may be educed from the comparison of the large body of facts which I have brought together as regards the classification of Foraminifera; and then briefly inquiring how far the results of similar comparisons, made upon other types of organized structure, justify the extension of the same principles to the Animal and Vegetable Kingdoms at large.

It may be well for me to advert in limine to certain peculiar features in this inquiry, that render the group to which it relates singularly adapted for a comparison at once minute and comprehensive amongst a wide range of individual forms. The size of the greater part of these organisms is so small, that many hundreds, thousands, or even tens of thousands of them, may be contained in a pill-box; and yet it is usually not too minute to prevent the practised observer from distinguishing the most important peculiarities of each individual by a hand-magnifier alone, or from dealing with it separately by a very simple kind of manipulation. Hence the systematist can easily select and arrange in series such of his specimens as display sufficient mutual conformity, whilst he sets apart such as are transitional or osculant; and an extensive range of varieties may thus be displayed within so small a compass, that the most divergent and the connecting forms are all recognizable nearly in the same glance. I am not acquainted with any other group of natural objects in which such ready comparison of great numbers of individuals can be made; and I am much mistaken if there be a single species of plant or animal, of which the range of variations has been studied by the collocation and comparison under one survey of so large an assemblage of specimens as have passed under review in the course of these investigations.

The general fact which I desire to bring prominently forward as the result of my investigations into this group, is, that in all the types possessing a wide geographical distribution which have been specially studied by myself, or by others, the range of variation has also been very wide; so that not only what have been considered as specific, but such as have been regarded as generic, and in some cases even as ordinal differences, present themselves among organisms, which, from the intimacy of the mutual relationship that is evinced by the gradational character of those differences, as well as by the variation presented in the several parts of one and the same organism, must in all probability have had a common origin.