Thus organs of different animals are said to be homologous when they are composed of like parts arranged in similar relation to one another. Homologous organs correspond bone for bone and tissue for tissue, so that each component of the one finds its respective counterpart in the other. The organs in question may be functionally specialized and externally differentiated for quite different purposes, but the superficial diversity serves only to emphasize, by contrast, the underlying identity of structure which persists intact beneath it. Thus, for example, the wing of a pigeon, the flipper of a whale, the foreleg of a cat, and the arm of a man are organs differing widely in function as well as outward appearance, but they are called homologous, none the less, because they all exhibit the same basic plan, being composed of similar bones similarly disposed with respect to one another. Organs, on the other hand, are called analogous which, though fundamentally unlike in structure, are, nevertheless, superficially modified and specialized for one and the same function. The wing of a bird and the wing of an insect furnish a trite instance of such analogy. Functionally they subserve the same purpose, but structurally they bear no relation to each other. In like manner, though both are devoted to the same function, there exists between the leg of a man and the leg of a spider a fundamental disparity in structure. At times, specialization for the selfsame function involves the emergence of a similar modification or uniform structural adaptation from a substrate of basic dissimilarity. In these instances of parallel modifications appearing on the surface of divergent types, we have something more than mere functional resemblance. Structure is likewise involved, albeit superficially, in the modification which brings about this external uniformity. In such cases, analogy is spoken of as _convergence_, a phenomenon of which the mole and the mole-cricket constitute a typical example. The burrowing legs of the insect are, so far as outward appearance goes, the exact replica on a smaller scale of those of the mole, though, fundamentally, their structure is quite unlike, the mole being built on the endoskeletal plan of the vertebrates, whereas the mole-cricket is constructed on the exoskeletal plan characteristic of the arthropods. Speaking of the first pair of legs of the mole-cricket, Thomas Hunt Morgan says: “By their use the mole-cricket makes a burrow near the surface of the ground, similar to, but of course much smaller than, that made by the mole. In both of these cases the adaptation is the more obvious, because, while the leg of the mole is formed on the same general plan as that of other vertebrates, and the leg of the mole-cricket has the same fundamental structure as that of other insects, yet in both cases the details of structure and the general proportions have been so altered that the