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Updated: May 26, 2025
The unchecked ingenuity of speculative anatomists proved itself fully competent to spin any number of contradictory hypotheses out of the same facts, and endless morphological dreams threatened to supplant scientific theory. Happily, however, there is a criterion of morphological truth, and a sure test of all homologies.
The philosophers had not at that time the arguments which might have been grounded on universal laws not yet suspected, such as the law of gravitation and the laws of heat; but there was a multitude, obvious even to them, of analogies and homologies in natural phaenomena, which suggested unity of plan; and a still greater number were raised up by their active fancy, aided by their premature scientific theories, all of which aimed at interpreting some phaenomenon by the analogy of others supposed to be better known; assuming, indeed, a much greater similarity among the various processes of Nature, than ampler experience has since shown to exist.
Between the ape and man, for example, there are over one hundred and fifty such anatomical homologies; that is, in the ape we find bone for bone, and muscle for muscle, corresponding to the structure of the human body. Even an animal so remotely related to man as the cat has many more resemblances to man in anatomical structure than dissimilarities.
The fact that such diverse points of view could direct the observer towards the same result gives, inferentially, a suggestive lesson in what the modern physiologist calls the homologies of parts of the organism. This classification, as will be observed, is not so very far afield from the modern divisions into mammals, birds, reptiles, amphibians, and fishes.
The formation of different languages and of distinct species, and the proofs that both have been developed through a gradual process, are curiously parallel. We find in distinct languages striking homologies due to community of descent, and analogies due to a similar process of formation. The manner in which certain letters or sounds change when others change is very like correlated growth.
The unchecked ingenuity of speculative anatomists proved itself fully competent to spin any number of contradictory hypotheses out of the same facts, and endless morphological dreams threatened to supplant scientific theory. Happily, however, there is a criterion of morphological truth, and a sure test of all homologies.
It may be said in opposition to the view that the insects came originally from the same early crustacean resembling the larva of a crab or shrimp, that the differences between the two types are too great, or, in other words, the homologies of the two classes too remote, and the two types are each too specialized to lead us to suppose that one was derived from the other.
In tracing the homologies of any part in different members of the same class, nothing is more common, or, in order fully to understand the relations of the parts, more useful than the discovery of rudiments. This is well shown in the drawings given by Owen of the leg bones of the horse, ox, and rhinoceros.
My work on the Cirripedia possesses, I think, considerable value, as besides describing several new and remarkable forms, I made out the homologies of the various parts I discovered the cementing apparatus, though I blundered dreadfully about the cement glands and lastly I proved the existence in certain genera of minute males complemental to and parasitic on the hermaphrodites.
In preparing these articles on the homologies of Radiates, I have felt the difficulty of divesting my subject of the technicalities which cling to all scientific results, until they are woven into the tissue of our every-day knowledge and assume the familiar garb of our common intellectual property.
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