further than this so that the hind legs have ceased to be apparent externally, and are only represented internally--and even this only in some species--by remnants so rudimentary that it is difficult to make out with certainty the homologies of the bones; moreover, the head and the whole body have become completely fish-like in shape. (Fig. 3.) But profound as are these alterations, they affect only those parts of the organism which it was for the benefit of the organism to have altered, so that it might be adapted to an aquatic mode of existence. Thus the arm, which is used as a fin, still retains the bones of the shoulder, fore-arm, wrist, and fingers, although they are all enclosed in a fin-shaped sack, so as to render them useless for any purpose other than swimming (Fig. 4.) Similarly, the head, although it so closely resembles the head of a fish in shape, still retains the bones of the mammalian skull in their proper anatomical relations to one another; but modified in form so as to offer the least possible resistance to the water. In short, it may be said that all the modifications have been effected with the least possible divergence from the typical mammalian type, which is compatible with securing so perfect an adaptation to a purely aquatic mode of life. [Illustration: FIG. 2.--Skeleton of Seal, 1/8 nat. size. Drawn from nature (_R. Coll. Surg. Mus._).] [Illustration: FIG. 3.--Skeleton of Greenland Whale, 1/100 nat. size. The rudimentary bones of the pelvis are shown on a larger scale in the upper drawing. (From Prof. Flower.)] [Illustration: FIG. 4.--Paddle of Whale compared with Hand of Man. Drawn from nature (_R. Coll. Surg. Mus._).] Now I have chosen the case of the whale and porpoise group, because they offer so extreme an example of profound modification of structure in adaptation to changed conditions of life. But the same thing may be seen in hundreds and hundreds of other cases. For instance, to confine our attention to the arm, not only is the limb modified in the whale for swimming, but in another mammal--the bat--it is modified for flying, by having the fingers enormously elongated and overspread with a membranous web. In birds, again, the arm is modified for flight in a wholly different way--the fingers here being very short and all run together, while the chief expanse of the wing is composed of the shoulder and fore-arm. In frogs and lizards, again, we find hands more like our own; but in an extinct species of flying reptile the modification was extreme, the wing having been formed by a prodigious elongation of the fifth finger, and a membrane spread over it and the rest of the hand. (Fig. 5.) Lastly, in serpents the hand and arm have disappeared altogether. [Illustration: FIG. 5.--Wing of Reptile, Mammal, and Bird. Drawn from nature (_Brit. Mus._).]