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Show 1892.] NUMERICAL VARIATION IN TEETH. 115 One or both of these propositions may be true. If the division of the other three first premolars were as complete as that of the left P^ there would be no indication of their origin. But if it is possible for a premolar to represent or to be represented by two premolars, without any visible indication of its double nature, may not the same be true of the premolars of other forms ? May it not be true of teeth generally ? And if it is true, how are the homologies of teeth to be determined ? Nevertheless teeth are almost preeminently amenable to this kind of treatment. They have been studied with immense care. The facts which they present, and on which their homologies are to be determined, are remarkably compact, and of all Series of Multiple Parts they offer the best chance. But examined in the light of a knowledge of the facts of Variation, that process is found to be capable of occurring in a way which precludes the possibility of carrying out an analysis of the relation between the parts and suggests that such relationship need not necessarily exist at all. This subject cannot now be discussed further; but if any one wishes to realize the difficulties suggested by the Variations of which instances have been given, let him read some good discussion of dental homologies, as, for example, Thomas's excellent paper l, with these cases in his mind, and as he reads let him ask himself what margin is left for the occurrence of phenomena like this. Such schemes as that alluded to, though they have done a most useful work, and though they are ingenious, logical, and orderly, are orderly because they are made without regard to the ways of Variation, which is arbitrary and capricious and follows no order that we have yet devised. An illustration will perhaps help to make clear the point at issue. The received view of homology supposes that a varying form is derived from the normal much as a man might make a wax model of the variety from a wax model of the type, by small additions to, and subtractions from, the several parts. This may, to our imaginations, seem, perhaps, the readiest way by which to make the varying form if we were asked to do it ; but the natural process differs in one great essential from this. For in nature the body of the varying form has never been the body of its parent and is not formed by a plastic operation from it; but in each case the body of the offspring is made again from the beginning, just as if the wax model had gone back into the melting-pot before the new model was begun. The present system of Homology must probably be retained as a basis of notation, imperfect though it is and though it is founded on a misconception of essential facts. It is likely that many will be disposed to doubt the reality of this misconception, and I can only ask that they should suspend judgment until the whole evidence can be produced. In the meantime this summary of facts and conclusions is put forward, together with a few " Prerogative Instances," in the hope that some one may be thereby attracted to a most powerful and fascinating method of zoological research. 1 Phil. Trans. 1887, vol. clxxviii.B, p. 443. 8* |
