SECT. VI.] 



INTRODUCTION. 



29 



It has thus been attempted to distinguish the Repetitions which 

 occur along the long axis of the body from those occurring 

 along the long axis of appendages, such as for example the joints 

 of antennae or of digits, and some have even gone so far as to 

 regard the Segmentation of the Vertebrate tail as a thing different 

 in kind from that of the trunk itself. It would be apart from our 

 present purpose to recur to these subjects, were it not that this 

 suggestion of the existence of a difference in kind between Meta- 

 meric Segmentation and other Repetitions has led to several 

 notable errors in the interpretation of the facts of morphology and 

 in the application of these facts to the solution of the problems of 

 Descent. In order to lay a sound foundation for the study of 

 Meristic Variation these errors must be cleared away, and to do 

 this it is necessary to break down the artificial distinction between 

 the phenomena of Metameric Segmentation and other cases of 

 Repetition of Parts, so that the whole may be seen in their true 

 relations to each other. When this is done, the mutual relations 

 of the facts of Meristic Variation will also become more evident. 



The first difficulty which has been brought into morphology 

 by the suggestion that Metameric Segmentation is a phenomenon 

 distinct in kind, is one which has coloured nearly all reasoning 

 from the facts of Morphology to problems of phylogeny. For the 

 existence of Metameric Segmentation in any given form is thus 

 taken to be one of its chief characters, and, as such, is allowed pre- 

 dominant weight in considering the genetic relations of these 

 forms. By the indiscriminate though logical extension of this 

 principle the conclusion has been reached that Vertebrates are 

 immediately connected with, or have arisen by Descent from 

 Annelids, or from Crustacea and the like, for the Repetition in 

 these forms is closely similar. Others again, being struck with 

 the resemblance between the Repetition of Parts along the radial 

 axes of Starfishes and those which occur along the long axes of 

 Annelids have hazarded the conjecture that perhaps this resem- 

 blance may indicate the actual phylogenetic history of these 

 Repetitions. Though such speculations as these are little better 

 than travesties of legitimate theory, some of them still command 

 interest if not belief 1 <^A.ll alike are founded on the assumption 

 that resemblances between the manner and degree in which Repe- 

 tition occurs are unlikely to have arisen save by community of 

 Descent^ A broader view of Meristic phenomena will shew that 



1 These modern "Instances" recall many that once were famous but are now 

 forgotten. For example: Item non absurda est similitudo et conformitaa ilia, ut 

 homo sit tanquam planta inversa. Nam radix nen-orum et facultatum animalium 

 est caput; partes autem seminales sunt infimce, non computatis extremitatibus tibi- 

 arum et brachiorum. At in planta, radix (quce instar capitis est) regulariter infimo 

 loco collocatur ; semina autem supremo. Bacon, Nov. Org. Lib. n. 27. In non 

 computatis extremitatibus, amateurs of Instants Confokmes may still find matter 

 for warning. 



