MORPHOLOGICAL RESEMBLANCE OR HOMOLOGY It is evident that the differences between those structures that lie distal to the wrist in the bird and mammal are associated with the different functions which they perform ; in the bird they have to support the largest feathers of the wing, in the other they have to be divided into fingers so as to enable the animal to gain a firm hold on the ground over which he is walking. It thus seems to be legitimate to regard the two structures as members of a single category, despite their difference in function. The nature of this group can be made clearer by a comparison of the wing of a bird with the parachute of the flying lizard "Draco," which performs similar functions in that it supports the animal in the air. In Draco the parachute consists of a fold of skin stretched out by a series of delicate unjointed bony rods which project to its margin. These rods are the animal's ribs each articulating independently with the backbone. There is in fact no structural resemblance between the two organs despite their functional similarity. The recognition of this difference between resem blances of structure and resemblances of function arose gradually and was only firmly established and defined by Richard Owen in 1843, who applied to morphological resemblance the term "homology" and to functional or resemblance, "analogy." In Owen's words, Homology "as the same organ in different animals under every variety of form and function (e.g., fore limbs of Draco volans and wings of a bird)" ; Analogy "as a part or organ in one animal which has the same function as an other part or organ in a different animal (e.g., parachute of `Draco' and wings of a bird)." This distinction between homologous and analogous structures is fundamental; it underlies all morphological studies whether they are concerned with comparative anatomy in the old sense, embryology, or the classification of animals.
Interpreted in evolutionary terms homologous structures are those which can be traced back to a single structure in the common ancestor of the animals compared. Thus if we knew the ancestors of a bird and a mammal for a sufficient distance back, we should see a steady divergence of structure of the fore limb, leading to the bird's wing and to the fore leg of a mammal. In fact the single fossil bird which is old enough to differ materially in struc ture from those now living, "Archaeopteryx," has three indepen dent fingers agreeing in the number of their phalanges and in their general structure with the first three fingers of a lizard, and we have evidence that the very remote ancestors of a mammal had a similar structure. Thus an investigation of their history confirms
the view that the two structures are homologous.
If homologous structures in different animals be derived by evolution from.the same structure in an ancestor, it should follow that the mode in which they arise during the development is the same in each case, and a new criterion of homology becomes avail able.
The limb bud gradually grows longer, its end becomes enlarged and divided into lobes which finally become fingers. This process is identical in bird and mammal for a considerable time.
If we compare a mammal with a dogfish we find a general similarity in the plan of structure. but whereas in one we have two pairs of appendages, the legs, lying at the extremities of the trunk, in the other legs are absent but two pairs of fins lie in a similar position. At first the structures of these two appendages, leg and fin, seem totally different; one has a bony skeleton sur rounded by muscles on all sides, whilst the other has a cartilag inous skeleton with the muscles lying only above and below it. The most obvious point of similarity is that in each case the nerves that supply the muscles come in the same way from the ventral branches of the spinal nerves. If we examine the mode of develop ment of the two organs we find a similarity in that each appears as a limb bud, a fold of skin lying longitudinally and sup ported by a mass of cells, but whereas in the mammal the muscles arise from these cells, in the fish they grow down from the body musculature. Thus even embryologically there are differences between the two structures.