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Updated: May 26, 2025
Now, the meaning of these anatomical homologies, biologists say, is that these animals are genetically related, that is, they had a common ancestry at some remote period in the past. The presence of vestigial organs in the higher animals supplies another argument for the belief in common descent.
In the great class of molluscs, though the parts in distinct species can be shown to be homologous, only a few serial homologies; such as the valves of Chitons, can be indicated; that is, we are seldom enabled to say that one part is homologous with another part in the same individual.
The botanists of Goethe's day could not see any sense, advantage, or practical application, to be made of the proposition that the parts of a blossom answer to leaves; and so the study of homologies had long to wait.
Further study, however, of the homologies of these peculiar appendages, and especially a knowledge of the embryological development of Lepisma and Machilis, is needed before this interesting point can be definitely settled. The three many jointed anal stylets may, however, be directly compared with the similar appendages of Perla and Ephemera.
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 new Transcendental Anatomy, the doctrine of Homologies, the Embryologic scheme, revealing that all animate forms are developed after one archetype, the splendid Nebular guess of Laplace, the thought of the Metamorphosis of Plants, the attempts at profounder explanations of Light and Colors, the rising transcendentalism of Chemistry, the magnificent intuition of Correspondence, showing a grand unity of design in the nodes of shells, the phyllotaxism of plants, and the serialization of planets, are all signs of the presence of a spirit that is to usher in a new dispensation of Science, fraught with divinest messages to the head and heart of man.
To return, however, to the subject of homological relations: it is surely inconceivable that indefinite variation with survival of the fittest can ever have built up these serial, bilateral, and vertical homologies, without the action of some special innate power or tendency so to build up, possessed by the organism itself in each case.
In the great class of molluscs, though we can homologise the parts of one species with those of another and distinct species, we can indicate but few serial homologies; that is, we are seldom enabled to say that one part or organ is homologous with another in the same individual.
On this same view of descent with modification, most of the great facts in Morphology become intelligible whether we look to the same pattern displayed by the different species of the same class in their homologous organs, to whatever purpose applied, or to the serial and lateral homologies in each individual animal and plant.
What are the homologies of this form and that? What its probable ancestry? What gaps does it bridge? What can it tell us of the story of animal creation? These and such like are the questions that have been ceaselessly before Haeckel's mind in all his studies of zoology.
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