SEGMENTATION 



69 



However their intermittence be determined, the rhythms of 

 division must be looked upon as the immediate source of those 

 geometrically ordered repetitions universally characteristic of 

 organic life. In the same category we may thus group the seg- 

 mentation of the Vertebrates and of the Arthropods, the concen- 

 tric growth of the Lamellibranch shells or of Fishes' scales, the 

 ripples on the horns of a goat, or the skeletons of the Foraminifera 

 or of the Heliozoa. In the case of plant-structures Church 4 has 

 admirably shown, with an abundance of detail, how on analysis 

 the definiteness of phyllotaxis is an expression of such rhythm in 

 the division of the apical tissues, and how the spirals and "ortho- 

 stichies" displayed in the grown plant are its ultimate conse- 

 quences. The problem thus narrows itself down to the question 

 of the mode whereby these rhythms are determined. 



It is natural that we should incline to refer them to a chemical 

 source. If we think of the illustration just given, of the seg- 

 mentation of a viscous fluid into drops by successive contractions 

 of a soft-walled tube we can, I think, conceive of such rhythmic 

 contractions as due to summations of chemical stimuli, somewhat 

 as are the beats of the heart. But when we recognize the vast 

 diversity of materials the distribution of which is determined by 

 an ostensibly similar rhythmic process it seems hopeless to look 

 forward to a directly chemical solution. That the chemical 

 degradation of protoplasm or of materials which it contains is 

 the source of the energy used in the divisions cannot be in dispute, 

 but that these divisions can be themselves the manifestations 

 of chemical action seems in the highest degree improbable. 



We may therefore insist with some confidence on the dis- 

 tinction between the Meristic and the substantive constitution 

 of organisms, between, that is to say, the system according to 

 which the materials are divided and the essential composition 

 of the materials, conscious of the fact that the energy of division 

 is supplied from the materials, and that in the ontogeny the 

 manner in which the divisions are effected must depend secon- 

 darily on the nature of the substances to be divided. The me- 



4 Church, A. H., On the Relation of Phyllotaxis to Mechanical Laws, London, 

 1904. 



