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Updated: June 3, 2025
Now, if such close relations of homology can be brought about independently of any but the most remote genetic affinity, it would be rash to affirm dogmatically that there is any impossibility in the independent origin of such forms as centetes and solenodon, or of genetically distinct batrachians, as similar to each other as are some of the frogs of South America and of Europe.
Now, as I have already said, the whole fabric of municipal law, of law within each nation, rests ultimately upon the judge and the policeman; and the complete absence of the policeman, and the almost complete absence of the judge, in international affairs, prevents there being as yet any real homology between municipal and international law.
Now, the presence of these useless bones and teeth is explained if they may be considered as actually being the inherited diminished representatives of parts of large size and functional importance in the remote ancestors of these various animals. Again, the singular facts of "homology" are capable of a similar explanation.
What can be more singular than the relation between blue eyes and deafness in cats, and the tortoise-shell colour with the female sex; the feathered feet and skin between the outer toes in pigeons, and the presence of more or less down on the young birds when first hatched, with the future colour of their plumage; or, again, the relation between the hair and teeth in the naked Turkish dog, though here probably homology comes into play?
It is in their limbs, however, that the Vertebrata present the most obvious and striking serial homology almost the only serial homology noticeable externally. The facts of serial homology seem hardly to have excited the amount of interest they certainly merit.
In the highest animals this symmetry is laid down at the very dawn of life, the first trace of the future creature being a longitudinal streak the embryonic "primitive groove." This kind of homology is explained by Mr. Spencer as the result of the similar way in which conditions affect the right and left sides respectively. It is much less general and marked than serial, or lateral homology.
Such are the facts of distribution in space and in time; the singular phenomena brought to light by the study of development; the structural relations of species upon which our systems of classification are founded; the great doctrines of philosophical anatomy, such as that of homology, or of the community of structural plan exhibited by large groups of species differing very widely in their habits and functions.
The evolutionary hypothesis here advocated also serves to explain all those remarkable facts which were stated in the first chapter to be explicable by the theory of Natural Selection, namely, the relation of existing to recent faunas and floras; the phenomena of homology and of rudimentary structures; also the processes gone through in development; and lastly, the wonderful facts of mimicry.
Such are the facts of distribution in space and in time; the singular phenomena brought to light by the study of development; the structural relations of species upon which our systems of classification are founded; the great doctrines of philosophical anatomy, such as that of homology, or of the community of structural plan exhibited by large groups of species differing very widely in their habits and functions.
They are the homology of the organs, and to a certain degree also the so-called rudimentary organs.
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