chap, i.] ARRANGEMENT OF EVIDENCE. 89 



results are differently disposed in the two cases. It is further to 

 be observed that the distinction, though striking, is nevertheless 

 one of degree, for the two kinds of Division pass gradually into 

 each other. By one or other of these two modes, or by a combin- 

 ation of both, all Meristic Series of Repetitions are formed. 



In Radial series, the Major Symmetry is built up by radial 

 divisions of the first kind, producing segments whose adjacent 

 parts are homologous, and related to each other as images. Each 

 of these segments is therefore bilaterally symmetrical about a 

 radial plane. There is no succession between the segments, and 

 in a perfectly symmetrical series, Successive or Linear repetitions 

 can only occur in Minor Systems of Symmetry. 



The considerations here set forth, though well known, have an 

 importance in the interpretation of the evidence, for the connexion 

 between the geometrical relations of organs and their Meristic 

 Variations is intimate. 



An arrangement of the facts with reference to these geometrical 

 relations cannot, of course, be absolute, for it is clear that a Bilateral 

 Symmetry, containing Linear Repetitions may be derived from a 

 Radial Symmetry, and that these figures cannot be precisely 

 delimited from each other ; nevertheless this plan of arrangement 

 has still several advantages. Chief among these is this : that it 

 brings out and emphasizes the fact that the possible, or at least 

 the probable Meristic Variations of such parts depend closely on 

 the geometrical relation in which they stand. This is, perhaps, in a 

 word, the first great deduction from the facts of Meristic Variation. 

 The capacity for, and manner of Meristic Variation appear to 

 depend not on the physiological nature of the part, on the system 

 to which it belongs, on the habits of the organism, on the needs 

 or exigencies of its life, but on this fact of the geometrical position 

 of the parts concerned. Linear series are liable to certain sorts of 

 Variation, Bilateral Series are liable to other sorts of Variation, 

 and Radial Series to others again. As I have ventured to hint 

 before, the importance of all this lies in the glimpse which is thus 

 afforded us of the essential nature of Meristic Division and 

 Repetition. Such interdependence between the geometrical re- 

 lations, or pattern, in which a part stands, and the kinds of 

 Variation of which it is capable, is, I think, a strong indication 

 that in Meristic Division we are dealing with a phenomenon 

 which in its essential nature is mechanical. Since this is a thing 

 of the highest importance, it will be useful to employ a system 

 which shall give it full expression. 



Evidence as to Meristic Variation in cell-division and in the 

 segmentation of ova will be spoken of in connexion with the Varia- 

 tion of Radial and Bilateral series. 



The second section of evidence is less immediately relevant to 

 the problem of Species; nevertheless it bears so closely on the 

 nature of Merism and on the mechanics of Physiological Division, 



