SEGMENTATION 63 



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 dif- 

 ferentiation 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 Phyl- 

 lopod series. In application to other cases however we soon meet 

 with difficulty, and the suggestion that the segments of a verte- 

 brate 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 com- 

 prehensible 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 com- 

 parative 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 



