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Updated: June 22, 2025
Hilaire has insisted strongly on the high importance of relative connexion in homologous organs: the parts may change to almost any extent in form and size, and yet they always remain connected together in the same order. We never find, for instance, the bones of the arm and forearm, or of the thigh and leg, transposed.
This resemblance is often expressed by the term "unity of type;" or by saying that the several parts and organs in the different species of the class are homologous. The whole subject is included under the general name of Morphology. This is the most interesting department of natural history, and may be said to be its very soul.
In drawing up the several series he trusts chiefly to embryological characters, but receives aid from homologous and rudimentary organs, as well as from the successive periods at which the various forms of life are believed to have first appeared in our geological formations. He has thus boldly made a great beginning, and shows us how classification will in the future be treated.
Mr. Lankester also adduces the close resemblance of the parts on the right and left sides of the body, and in the successive segments of the same individual animal; and here we have parts commonly called homologous which bear no relation to the descent of distinct species from a common progenitor.
And this, strange as it may appear at the first glance, seems to me scarcely doubtful. It is true that the act of adhesion of the Rhizocephala has never yet been observed, but it is more than probable that they attach themselves, just like the Cirripedia, by means of the antennae, and that therefore the points of attachment in the two groups indicate homologous parts of the body.
"Now, on one occasion, in a flower belonging to an individual of Salvia verticillata, and only on the left stamen, I observed a perfectly developed and pollinigerous lower cell, perfectly homologous with that which is normally developed in Salvia officinalis. This case of atavism is truly singular.
The formula of conine, C H N, shows it to be homologous with piperidine, C H N, a derivative of piperine, the alkaloid of pepper, to be spoken of later; and, just as piperidine is derived from pyridine by the action of reducing agents, so conine is probably derived from a propyl-pyridine. The artificial alkaloid paraconine, isomeric with the natural conine, will be referred to later.
I know a moth suffers most over any injury to them; but one takes flight no quicker or more precipitately at a touch on the antennae than on the head, wing, leg, or abdomen. We are safe in laying down a law that antennae are homologous organs and used for identical purposes on all forms of life carrying them. The short antennae of grasshoppers appear to be organs of scent.
When this spontaneous faculty of man has been developed within him, his observation of the similarities, analogies, differences, and identities which are to be found in all things and phenomena, in sentiments and emotions, necessarily induces him to collect and simplify them in special forms, to combine these various intuitions in a homologous type; this type corresponds with an external or internal congeries of similar, identical, or analogous images or ideas, out of which the species and genera of the intellect are formed.
Most physiologists believe that the bones of the skull are homologous with that is correspond in number and in relative connexion with the elemental parts of a certain number of vertebræ. The anterior and posterior limbs in each member of the vertebrate and articulate classes are plainly homologous. We see the same law in comparing the wonderfully complex jaws and legs in crustaceans.
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