In the freedom of nature such superficial variations are not so obvious. But among the invertebrates they are not inconsiderable. The case of land-snails, already quoted, may again be cited. Taking variations in banding alone, Mr. Cockerell knows of 252 varieties of _Helix nemoralis_ and 128 of _H. hortensis_. Still, among the wild relatives of our domestic breeds of animals and birds the superficial variations are decidedly less marked. And this is partly due to the fact that they are in a state of far more stable equilibrium than our domestic products, and partly to the constant elimination of all variants which are thereby placed at a serious or vital disadvantage. White rats, mice, or small birds, in temperate regions, would soon be seized upon by hawks and other enemies. If the eggs and young of the Kentish plover, shown in our frontispiece, were white or yellowish, like the eggs and young of our fowls, they would soon be snapped up. The varied protective resemblances, general and special, have been brought about by the superficial variations of organisms, and the elimination of those which, from non-variation or wrong variation, remained conspicuous. We need only further notice one thing here, namely, that, in the case of special resemblance to an inorganic object or to another organism, the variations of the several parts must be very closely, and sometimes completely, correlated. The correlations, however, need not, perhaps, have been simultaneous--the resemblance having been gradually perfected by the filling in of additional touches, first one here, then another there, and so on. Concerning "organic variations," little need be said. It is clear that an organ or limb may vary in size, such variation carrying with it a correlative variation in power; or it may vary in complexity--the teeth of the horse tribe, for example, having increased in complexity, while their limbs have been rendered less complex; or it may vary in efficiency through the more perfect correlation and co-ordination of its parts. The evidence of such variations from actual observation is far less in amount than that of superficial variations. And this is not to be wondered at, since in many cases it can only be obtained by careful anatomical investigation. Nevertheless, anatomists, both human and comparative, are agreed that such variations do occur. And no one can examine such a collection as that of the Royal College of Surgeons without acknowledging the fact. Thirdly, "reproductive and developmental variations" are of very great importance. The following are among the more important modifications which may occur in the animal kingdom. 1. Variations in the mode of reproduction, sexual or asexual. 2. Variations in the mode of fertilization. 3. Variations in the number of fertilized ova produced. 4. Variations in the amount of food-yolk and in the way in which it is supplied. 5. Variations in the time occupied in development.