If, then, the theory of genera be such as I have endeavoured to expound, it results from what has been said, _that every generic type is to be looked for in, or about, the centre of its peculiar group_,--or at any rate in that region of it which would seem to be the most characteristically, or evenly, pronounced. I lay particular stress upon this conclusion, because (if correct) it will somewhat modify the notions which are occasionally entertained upon the subject. A stricture, however, may here be required upon what I have advanced, lest, through using the metaphors _which I selected for the elucidation of a principle_, it be supposed that I would wish them to apply to the smaller details, likewise, of the problem. If a genus has been portrayed under the similitude of a bulb, or of a nodule (formed by the approximation of beads which more or less resemble each other in their primary aspect), it does not follow that either bulb or nodule are to diminish in a similar ratio towards their respective circumferences,--or, which is the same thing, that they are to be symmetrical; whether spherical, ovoid, or otherwise. The general method of the organic creation is a progressive one; and its successive types, therefore, will not always be found to radiate _equally_ from their normal foci: so that it is in the direction of the _higher_ (rather than the lower) extremities of the assemblages that those foci are usually to be discerned;--and where the groups are large, it is not often difficult to pronounce which of their ends are, as a whole, the more perfectly developed. It will, moreover, be further acknowledged (if my premises are allowed), that, since it is a somewhat central position which the typical member of a genus usually occupies, _the diagnostic characters_, although (in combination) carried out to the full, _are more evenly balanced in a generic type than in any of its associates_; or, in other words, that a species in which any single organ is monstrously enlarged, at the expense of the rest, is seldom typical of the assemblage with which it is placed; but may be _a priori_ regarded as in all probability a transition form, leading us onwards into some neighbouring group[81].