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CHAPTER VIII.

HOMOLOGIES.

Animals made-up of parts mutually related in various ways. —What homo

logy is.—Its various kinds. --Serial homology.-Lateral homology.Vertical homology.--Mr. Herbert Spencer's explanations. —An internal power necessary, as shown by facts of comparative anatomy.—Of teratology.-M. St. Hilaire.—-Professor Burt Wilder.—Foot-wings. - Facts of pathology.—Mr. James Paget. - Dr. William Budd.—The existence of such an internal power of individual development diminishes the improbability of an analogous law of specific origination.

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That concrete whole which is spoken of as an individual” (such, e.g., as a bird or a lobster) is formed of a more or less complex aggregation of parts which are actually (from whatever cause or causes) grouped together in a harmonious interdependency, and which have a multitude of complex relations amongst themselves.

The mind detects a certain number of these relations as it contemplates the various component parts of an individual in one or other direction—as it follows up

different lines of thought. These perceived relations, though subjective, as relations, have nevertheless an objective foundation as real parts, or conditions of parts, of real wholes ; they are, therefore, true relations, such, e.g., as those between the right and left hand, between the hand and the foot, &c.

The component parts of each concrete whole have also a relation of resemblance to the parts of other concrete wholes, whether of the same or of different kinds, as the resemblance between the hands of two men, or that between the hand of a mau and the fore-paw of a cat.

Now, it is here contended that the relationships borne one to another by various component parts, imply the existence of some innate, internal condition, conveniently spoken of as a power or tendency, which is quite as mysterious as is any innate condition, power, or tendency resulting in the orderly evolution of successive specific manifestations. These relationships, as also this developmental power, will doubtless, in a certain sense, be somewhat further explained as science advances. But the result will be merely a shifting of the inexplicability a point backwards, by the intercalation of another step between the action of the internal condition or power and its external result. In the meantime, even if by “ Natural Selection” we could eliminate the puzzles of the "origin of species,” yet other phenomena, not less remarkable (namely, those noticed in this chapter), would still remain unexplained and inexplicable. It is not improbable that, could we arrive at the causes conditioning all the complex inter-relations between the several parts of one animal, we should at the same time obtain the key to unlock the secrets of specific origination.

It is desirable, then, to see what facts there are in animal organization which point to innate conditions (powers and tendencies) as yet unexplained, and upon which the theory of “Natural Selection” is unable to throw any explanatory light.

The facts to be considered are the phenomena of “homo

logy," and especially of serial, bilateral, and vertical homology.

The word “homology” indicates such a relation between two parts that they may be said in some sense to be “the same," or at least “of similar nature.” This similarity, however, does not relate to the use to which parts are put, but only to their relative position with regard to other parts, or to their mode of origin. There are many kinds of homology, but it is only necessary to consider the three kinds above enumerated.

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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. But the arm of man, the foreleg of the horse, the paddle of the whale, and the wing of the bat and of the bird, are all

1 For an enumeration of the more obvious homological relationships see Ann. and Mag. of Nat. Hist. for August 1870, p. 118.

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likewise homologous parts, yet of another kind; that is they are the same parts existing in animals of different species.

In contrast to this, the wing of the humming-bird and the wing of the humming-bird moth are not homologous at all, or in any sense; for the resemblance between them consists solely in the use to which they are put, and is

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(Showing the elongated ribs which support the flitting organ.)

therefore only a relation of analogy. There is no relation of homology between them, because they have no common resemblance as to their relations to surrounding parts, or as to their mode of origin. Similarly, there is no homology between the wing of the bat and that of the flying-dragon, for the latter is formed of certain ribs, and not of limb bones.

Homology may be further distinguished into (1) a relationship which, on evolutionary principles, would be due to descent from a common ancestor, as the homological

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TARSAL BONES OF DIFFERENT LEMUROIDS. ig ht-hand figure, Tarsus of Galago ; left-hand figure, Tarsus of Cheirogaleus.)

relation between the arm-bone of the horse and that of the ox, or between the singular ankle-bones of the two lemurine genera, cheirogaleus and galago, and which relation has

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