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Updated: June 22, 2025
The little mouse hands his name down through the years from the old, old Sanskrit, the root meaning to steal. Many people who never heard of Sanskrit have called him and his descendants by terms of homologous significance!
On the older view the hairs of the mammals are equivalent or homologous to the feathers of the bird or the horny scales of the reptile. In passing from aquatic to terrestrial life the horny scales were further developed, and the bony scales degenerated in most of the reptiles.
Such parts, whether more or less modified, unless their common origin became wholly obscured, would be serially homologous.
In passing to the figure of the bird shown in plate CXXXVIII, b, we find features homologous with those already considered, but also detect considerable modification. The head is elongated, tipped with three parallel lines, but decorated with markings similar to those of the preceding figure.
Hence the same names can be given to the homologous bones in widely different animals. We see the same great law in the construction of the mouths of insects: what can be more different than the immensely long spiral proboscis of a sphinx-moth, the curious folded one of a bee or bug, and the great jaws of a beetle?
Mountain breeds always differ from lowland breeds; and a mountainous country would probably affect the hind limbs from exercising them more, and possibly even the form of the pelvis; and then by the law of homologous variation, the front limbs and even the head would probably be affected. The shape, also, of the pelvis might affect by pressure the shape of the head of the young in the womb.
The several parts which are homologous, and which, at an early embryonic period, are identical in structure, and which are necessarily exposed to similar conditions, seem eminently liable to vary in a like manner: we see this in the right and left sides of the body varying in the same manner; in the front and hind legs, and even in the jaws and limbs, varying together, for the lower jaw is believed by some anatomists to be homologous with the limbs.
Here, then, is a source of replenishment as lasting as the sun itself, and a ground so far as a supply of force is concerned for indefinite duration. Perhaps, however, M. Naudin does not mean that "evolutive force," or the force of vitality, is really homologous with common physical force, but only something which may be likened to it.
The conception of latency of characters as the common source of the origination of varieties, either in the positive or in the negative way, leads to some rules on variability, which are known under the names given to them by Darwin. They are the rules of repeated, homologous, parallel and analogous variability.
Turner, has been expressly described by Theile as a rudimentary repetition of the extensor of the tail, a muscle which is so largely developed in many mammals. The upper part of this filament, as Prof. Turner informs me, is undoubtedly homologous with the spinal cord; but the lower part apparently consists merely of the pia mater, or vascular investing membrane.
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