chap. I.] ARRANGEMENT OF EVIDENCE. 87 



been a progress from a more fully segmented form to forms less seg- 

 mented, I would again point out that this view is in direct opposition 

 to the indications afforded by the lower Chordata, which are less and 

 not more segmented than the higher forms. 



The hypothesis of an ancestor made up of complete segments is 

 resorted to because it is felt to be difficult to conceive the progressiva 

 building up of a segmented form, but on appeal to the facts of Variation 

 the evidence will clearly shew that Repetition of parts previously (wilt- 

 ing is a quite common phenomenon; that such repetition may occur in 

 almost any system of organs; and lastly that such new repetitions may 

 be coincident in the several systems. To argue moreover that these 

 repetitions, for instance that of oviducal apertures in Astacus, of 

 mammas or cervical ribs in mammals are "reversions," leads to ab- 

 surdity, for on the same reasoning, the occurrence, in the Crab, of a 

 third maxillipede formed as a chela, would shew that these appendages 

 had been originally chehe, that the occurrence of petaloid sepals shews 

 that the sepals had originally been petals, and so forth. 



These considerations will suffice to illustrate the great difference 

 of degree, if not of kind, which probably exists between these two 

 kinds of segmentation, that which arises by the repetition of bud- 

 like segments, each containing parts of many systems on the one 

 hand, and the progressive and separate segmentation of the several 

 systems on the other. For reasons already given, however, I shall 

 not attempt in this first collection of evidence to separate the facts 

 on these lines. Though some cases can at once be seen to be 

 strictly Meristic while others are plainly Homceotic, many cannot 

 be affirmed to belong to the one group rather than to the other. 

 There is, besides, a serious doubt whether perhaps after all, 

 Homceotic Variation even in its most marked forms, may Dot 

 ultimately rest on and be an expression of a change in the pro- 

 cesses of Division, and be thus, at bottom, strictly Meristic also. 

 In our present ignorance of the physics of Division, this doubt 

 cannot be satisfied, and therefore it will be best to make no 

 definite separation between the two classes of variations, though 

 whenever the nature of a given variation is such that it may at 

 once be recognised as Homceotic, it will be well to specify this. 



In the absence of a more natural classification, the material 

 has been roughly arranged with reference to the geometrical 

 disposition and relations of the structures concerned. In the 

 Introduction, Section IV. p. 21, reference was made to the (aft 

 that the Symmetry of an organism may be such as to include all 

 the parts into one system of Symmetry, and for such a system the 

 term Major Symmetry was proposed. Systems of this kind are 

 seen in the Vertebrates and Echinoderms, for example. On the 

 other hand systems of Symmetry occur in limbs and other separate 

 parts of organisms, in such a way that each such system is either 

 altogether or partially geometrically complete and symmetrical in 

 itself. For example, the toe of a Horse, the arm of a Starfish, the 



