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Updated: June 3, 2025
By homology of organs we mean the fact that within one and the same class-group of organisms all the organs, and especially the organs in their most solid constituents, in the skeleton, are built after one and the same fundamental plan, and therefore are even in their most widely separated modifications varied after this one and the same plan.
Spencer's assertion, however, would require the bringing forward of examples of organisms which are ill-adapted to their positions, and out of harmony with their surroundings a difficult task indeed. Secondly, as regards the last-mentioned author's explanation of such serial homology as exists in the centipede and its allies, the very groundwork is open to objection.
"Homology" is the name applied to the investigation of those profound resemblances which have so often been found to underlie superficial differences between animals of very different form and habit.
These productions, so different in their origin, can only be compared from the point of view of the part they play; there are analogies between them but not the least homology. Animals naturally provided with dwellings.
What can be more singular than the relation between blue eyes and deafness in cats, and the tortoise-shell colour with the female sex; the feathered feet and skin between the outer toes in pigeons, and the presence of more or less down on the young birds when first hatched, with the future colour of their plumage; or, again, the relation between the hair and teeth in the naked Turkish dog, though here probably homology comes into play?
Thus, then, there is something to be said in opposition to both the aggregational and the mechanical explanations of serial homology. The explanations suggested are very ingenious, yet repose upon a very small basis of fact. Not but that the process of vertebral segmentation may have been sometimes assisted by the mechanical action suggested.
"I am expressing," he said, "something more than an analogy, I am describing a real homology so far as concerns the process of development, when I say that these communities simulated modern European nations, much in the same way that a tree-fern of the carboniferous period simulated the exogenous trees of the present time." So far as this is true of China, it is likewise true of Japan.
That the idea of transmutation of parts, as well as of mere homology, was in mind is evidenced by a very remarkable sentence in which Aristotle asserts, "Empedocles says that fingernails rise from sinew from hardening."
Homology may be further distinguished into a relationship which, on evolutionary principles, would be due to descent from a common ancestor, as the homological relation between the arm-bone of the horse and that of the ox, or between the singular ankle bones of the two lemurine genera, cheirogaleus and galago, and which relation has been termed by Mr.
Such are the facts of distribution in space and in time; the singular phenomena brought to light by the study of development; the structural relations of species upon which our systems of classification are founded; the great doctrines of philosophical anatomy, such as that of homology, or of the community of structural plan exhibited by large groups of species differing very widely in their habits and functions.
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