556 MERISTIC VARIATION. [part I. 



That this is so may, I think, be in part at least attributed to 

 the normal twisting of the vertebrate limb, especially of the hind 

 limb, from its original position (see Note on p. 459). 



A question brought into prominence by facts of this kind 

 is that of the nature of the control which determines how mucli 

 of a body shall be repeated, or be capable of repetition, in a 

 Secondary Symmetry. 



What is a unit of repetition ? 



With repetition of a whole body we are familiar. Apart from 

 the processes of sexual reproduction, we know this total repetition 

 in the many forms of asexual reproduction, whether occurring 

 by budding, or by division either of adult bodies or of embryos 1 , 

 and we thus commonly look on the whole body of any organism 

 as in a sense a unit, capable of repetition or of differentiation — the 

 latter especially in gregarious and colonial forms. Again, we 

 familiarly use the conception of cells as units of repetition or of 

 differentiation. Besides these we have come to recognize that 

 members of series of segments are, in their degree, similar units. 

 And generally, the same attribute of separateness may in un- 

 defined senses be properly attached to all organs that are re- 

 peated in Series, and to appendicular parts especially. 



The attribution of some of the undefined properties of "unity 2 " 

 to some at least of these various groups is very ancient, and there 

 can be no doubt that it is in the main a right and useful in- 

 duction. 



The chief interest of repetitions in Secondary Symmetry lies 

 in the fact that they give a glimpse of new light upon the nature 

 of this unity, shewing a new form in which it may appear. 



For in Secondary Symmetry there is not a simple repetition 

 of a part in Series, taking its place as a member of that series, 

 but an addition of paired parts, whose intrinsic relation to each 

 other is the same as that of any pair of parts occurring in the 

 Primary Symmetry. 



The addition is thus a unit, is in form complete in itself, and 

 seems to have no place in the Primary Symmetry of the whole 

 body any more than a late side-chapel — also a unit with its own 

 focus and polarity — had a place in the design of t lie original archi- 

 tect of the Cathedral. 



From analogy, and from general knowledge of vital processes 

 it would I think have been impossible to foresee the very curious 

 indeflniteness of the quantity of the parts repeated in systems 

 of Secondary Symmetry. It seems, especially in Arthropod cases, 



1 As a normal occurrence notably in the case of Cyclostomatous Polyzoa of the 

 genus Grisia described by Harmer, S. F., Q. J. M. S., 1891, p. 127, Plates. 



2 This somewhat incorrect term is used here to express some of the meanings 

 commonly still more incorrectly rendered by the word "individuality" — a word 

 etymologically most unhappy in this application to things endowed with divisibility 

 as a conspicuous attribute. 



