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Show 114 MR. W. BATESON O N [Feb. 2, that any one of these teeth should be considered supernumerary more than any other ; and in the absence of such evidence it would, as I think, be best to regard the four premolars in this specimen as collectively representing the three premolars of the normal. For just as a stick may be broken into three pieces or into four, so would it seem to be with Multiple Parts. The epithelium which normally gives off three enamel-germs has here given off four such germs, and I believe that it is as impossible to analyze the four teeth and apportion them out among the three teeth as it would be to homologize the sides of an equilateral triangle with the sides of a square of the same peripheral measurement, or to homologize the segments of a 4-rayed Sarsia 1 with segments of its 6-rayed " sport." To make such an attempt would be to disregard the plain and obvious indications of the true nature of the phenomenon, and any theory of Homology which recognizes this class of problem as profitable or legitimate is, I believe, founded upon a wrong conception of the physical process of Division. For, after all, it is with a process of physiological Division that we have here to do, and the Division which results in the formation of a series of Multiple Parts is probably a manifestation of the same physical process as the Division of a cell or the segmentation of an ovum. Whoever will discover by what physical process an ovum segments will give us the key to the problem of the segmentation of tissues into Series of Multiple Parts; and though we are far enough from having any such knowledge, we should at least recognize that this is the problem to be dealt with, and any working hypothesis of the nature of Homology should be, at all events, in harmony with what is known of the processes of Division and should be founded upon them. Now the ordinarv conception of the relationship of Homology as defined above, though it has been a useful instrument as a basis of nomenclature and so forth, is nevertheless inconsistent with the facts of Division and is founded on assumptions which are not justified, suggesting a view of the physics of Division which is wrong. In order to appreciate this, let the reader consider, for example, the case of Ommatophoca rossii given above. Judged by the ordinary rules of morphological criticism, this specimen shows one or both of two things :- (1) The first premolar of Ommatophoca may in itself represent two premolars of an ancestor. Or (2) In the descendants of Ommatophoca the single first premolar may be represented by two distinct and several premolars. 1 Sarsia is the Medusa of a Gymuoblastic Hydroid (Syncoryne). The normal form has 4 radial canals, 4 ocelli, and 4 tentacles. Three specimens having six of each of these parts are recorded, two (American) by L. Agassiz, Mem. Amer. Acad. Sci. iv. 248, pi. v. fig. 5, and one (British) by Romanes! Journ. Linn. Soc. xii. p. 527. All of these were radially symmetrical. |
