same general plan of organization, we find many organs having the same embryonic origin and the same general relations to other structures, but with vastly different superficial appearance and playing quite diverse functional roles. Such structures are said to be homologous.
A common example of homologous structures is presented by the forelimbs of various types of backboned animals (vertebrates); such, for example, as that of man, that of the whale, that of the bird and that of the horse. The arm of man is by far the most generalized of these; it is not far from the ideal prototypic land vertebrate fore limb, and in that it is not specialized for any particular function, but is a versatile tool of the brain. The flipper of the whale is a short, broad, paddlelike structure, apparently without digits, wrist, forearm or upper arm; but on close examination it is seen to possess all of these structures in a condition homologous, almost bone for bone and muscle for muscle, with those of the human arm. The wing of the bird, a highly specialized organ of flight, appears superficially to have nothing in common with the arm of man; but a study of its anatomy shows the same bony architecture and muscle complex, modified rather profoundly for a different function and with the thumb and two of the fingers greatly reduced or entirely unrepresented in the adult stage. The fore-leg of the horse is a specialized cursorial appendage, and in accord with this function has bunt one functional toe with a heavy toenail or hoof. Two other toes are represented by the so-called split bones, mere vestiges of once useful structures. In other respects the horse's leg is quite homologous with that of other land vertebrates. The evolutionary explanation of the fact that these several types of limbs (each playing an entirely different role in nature and each so unlike the other in form and proportions) have the same fundamental architecture, is that they have all inherited these characters from some distant common ancestor. In each case the inheritance has undergone modification in harmony with the life needs of the organism. This, of course, implies descent with modification, which is no more or less than evolution.
An equally significant situation comes to light in connection with the hind-limbs of vertebrates. The leg of man, a specialized walking appendage, is much less versatile than is the arm; yet it is closely homologous with the latter. The hind-limb of the whale is, in some species, entirely wanting in the adult or else is in vestigial condition. The leg of the bird is decidedly reptilian in structure and is believed to have retained, in large measure, the characteristics of that of the supposed reptilian ancestors. The hind-limb of the horse, though somewhat stronger and heavier than the fore-limb, resembles the latter closely both in form and function. Snakes are typically limbless vertebrates, but the python has small but clearly defined hind-limbs, somewhat reduced in number of bones and almost entirely hidden beneath the scaly integument.
No other attempt to explain homologies such as those briefly outlined above has been made except that of special creation, and this implies a slavish adherence to a preconceived ideal plan together with capricious departures from the plan in various instances. A systematic attempt to apply the special creation concept to all cases of homologies involves one in the utmost confusion of ideas and leads almost inevitably to irreverence, which is abhorent to evolutionists as well as to special creationists.
Vestigal Structures.
These may be defined in functionless rudiments of structures whose homologues are found in a functional state in other members of a group with a common architectural plan. Thus the hind-limbs of the whale and of the python, the thumb of the bird, the split bones of the horse, are vestigal homologues of structures well developed in more generalized groups of vertebrates.
