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Very few writers, however, have devoted much time or thought to the question of serial homology in general. Mr. Herbert Spencer, indeed, in his very interesting "First Principles of Biology," has given forth ideas on this subject, which are well worthy careful perusal and consideration, and some of which apply also to the other kinds of homology mentioned above.
This subject of comparative anatomy includes a consideration of what is called homology, and perhaps a concrete example may be instructive both in illustration and as suggesting the course which nature adopts in constructing her machines. We speak of a monkey's arm and a bird's wing as homologous, although they are wonderfully different in appearance and adapted to different duties.
If, however, other reasons against it appear of any weight if, especially, there is reason to believe that geological time has not been sufficient for it, then it will be well to bear in mind the facts here enumerated. Animals made-up of parts mutually related in various ways. What homology is. Its various kinds. Serial homology. Lateral homology. Vertical homology. Mr.
Therefore homology of organs is no proof of their hereditary affiliation. They rather illustrate “iterative evolution.”
Especially in our day, when it became apparent that the actual use of an organ might not be the fundamental reason of its existence that one and the same organ, morphologically considered, was modified in different cases to the most diverse uses, while intrinsically different organs subserved identical functions, and consequently that use was a fallacious and homology the surer guide to correct classification it was not surprising that teleological ideas nearly disappeared from natural history.
There are many kinds of homology, but it is only necessary to consider the three kinds above enumerated. The term "homologous" may be applied to parts in two individual animals of different kinds, or to different parts of the same individual. Thus "the right and left hands," or "joints of the backbone," or "the teeth of the two jaws," are homologous parts of the same individual.
For what is a metaphor or a simile but a mere paralogism having nothing to do with the matter in hand, and not to be allowed for a moment to influence the reader's judgment, unless there be some real and objective analogy homology we should call it between the physical phenomenon from which the symbol is taken, and the spiritual truth which it is meant to illustrate?
In all these particulars, the mandible of Lepisma is comparable with that of certain Coleoptera and Neuroptera. So also are the maxillæ and labium, though we are not aware that any one has indicated how close the homology is. The accompanying figure of the maxilla of a beetle may serve as an example of the maxilla of the Coleoptera, Orthoptera and Neuroptera.
If, assuming descent, this homology were disturbed, and the systematic relationship done away with, for instance through saltatory evolution, the mere fact of descent would not bring the two species any nearer one another. Thus the case proves only systematic relationship, and only evolution.
Thirdly, and lastly, as regards such serial homology as is exemplified by the backbone of man, there are also several objections to Mr. Spencer's mechanical explanation. On the theory of evolution most in favour, the first Vertebrata were aquatic.
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