Similar questions arise whenever parts are repeated in series, whether the series be linear or radial, and, though less obviously, even when the repetition is bilateral only. In each such example the question arises, is the resemblance between the parts the remains of a still closer resemblance, or is differentiation original? Sometimes the view that these parts have arisen by the differentiation of a series of identical parts is plausible enough, as for example when the peculiarities of various appendages of a Decapod Crustacean are referred to modifications of the Phyllopod series. In application to other cases however we soon meet with difficulty, and the suggestion that the segments of a vertebrate were originally all alike is seen at once to be absurd, for the reason that a creature so constituted could not exist, and that, differentiation of at least one anterior and one posterior segment, is an essential condition of a viable organism consisting of parts repeated in a linear series. Between these two terminal segments it is possible to imagine the addition of one segment, or of a series of approximately similar segments; but when once it is realised that the terminals must have been differentiated from the beginning, it will be seen that the problem of the origin of the resemblance between segments is not rendered more comprehensible by the suggestion that even the intervening members were originally alike. Seeing indeed that some differentiation must have existed primordially it is as easy to imagine that the original body was composed of a series grading from the condition of the anterior segment to that of the posterior, as any other arrangement. The existence of a linear or successive series in fact postulates a polarity of the whole, and in such a system the conception of an ideal segment containing all the parts represented in the others has manifestly no place. The introduction of that conception though sanctioned by the great masters of comparative anatomy, has, as I think, really delayed the progress of a rational study of the phenomena of division. The same notion has been applied to every class of repetition both in animals and plants, generally with the same unhappy results. In the cruder forms in which this doctrine was taught thirty years ago it is now seldom expressed, but modified presentations of it still survive and confuse our judgments. The process of repetition of parts in the bodies of organisms is however a periodic phenomenon. This much, provided we remain free from prejudice as to the nature and causation of the period or rhythm, we may safely declare, and a comparison may thus be instituted between the consequences of meristic repetition in the bodies of living things and those repetitions which in the inorganic world are due to rhythmical processes. Of such processes there is a practically unlimited diversity and we have nothing to indicate with which of them our repetitions should rather be compared.