28 METAMERIC SEGMENTATION. [introd. 



to be included under the term Meristic. With variations in 

 quality and Substance it is not at present proposed to deal, except 

 in so far as it is necessary to refer to them in their relation to the 

 phenomena of Merism, and in illustration of the structural possi- 

 bilities or necessities which in the body follow as corollaries upon 

 the existence of Meristic Repetition. 



It has also been proposed to limit the consideration to Varia- 

 tions which are Discontinuous. As has been already stated, Dis- 

 continuous Variations may belong to the Meristic Group or to the 

 Substantive, but it is to the former that attention will first be 

 directed. 



SECTION VI. 

 Meristic Repetition and Homology. 



In what has gone before, the two conceptions now introduced, 

 namely the distinction of Variations into Meristic and Substan- 

 tive, and into Continuous and Discontinuous, have been sketched 

 in outline. The significance of the facts which follow will be 

 made more evident if these two conceptions are now more fully 

 developed in some of their aspects. 



Under the name Merism I have proposed to include all pheno- 

 mena of Repetition and Division, whenever found and in whatever 

 forms occurring, whether in the parts of a body or in the whole. 

 The consequences of the admission of this proposition are con- 

 siderable and should be fully realized ; for on recognition of the 

 unity of these phenomena it is possible to group together a number 

 of facts whose association will lead to simplification of some 

 morphological conceptions, and to other results of utility. 



That the phenomena of Merism form a natural group is in 

 some respects a familiar idea, but in its fullest expression it is as 

 yet not generally received, still less have the consequences which 

 it entails been properly appreciated. Every one who has gone 

 even a little way into morphological inquiry has met some of the 

 difficulties to which we shall now refer. 



It is with respect to the phenomena of Segmentation that 

 these difficulties are most familiar, and it is in this connexion that 

 they may be best discussed. Segmentation is a condition which 

 reaches its highest development in Vertebrates, the Annelids, 

 and the Arthropods, and it is in these groups that it has been 

 most studied. In them it appears as a more or less coincident 

 Repetition of elements belonging to most of the chief systems 

 of organs along an axis corresponding to the long axis of the body. 

 To segmentation of this kind the name ' Metameric ' has been 

 given, and by many morphologists the attempt has been made, 

 either tacitly or in words, to separate such Metameric Segmen- 

 tation from other phenomena of Repetition elsewhere occurring. 



