20 MEFJSTIC REPETITION. [introd. 



little doubt that Symmetry is present, though owing to the slight 

 degree of Differentiation, its presence may not be clearly 

 perceived. In the manifestations, however, in which it is most 

 familiar, Symmetry is a decided and obvious phenomenon. 



Symmetry then depends essentially on the fact that structures 

 found in one part of an organism are repeated and occur again in 

 another part of the same organism. Symmetrical Heterogeneity 

 may therefore be present in a spherical body having a core 

 of different material, and it is possible that in an unsegmented 

 ovum for example a Symmetry of this simple kind may exist. 

 But Symmetry, as it is generally seen in organisms, differs from 

 that of these simplest cases in the fact that the organs repeated 

 are separated from each other by material of a nature different 

 from that of the organs separated. Repetitions of this kind are 

 known in almost every group of animals and plants. The parts 

 thus separated may belong to any system of organs. There is no 

 known limit to the number of Repetitions that may occur. 



This phenomenon of Repetition of Parts, generally occurring 

 in such a way as to form a Symmetry or Pattern, comes near to 

 being a universal character of the bodies of living things. It will 

 in cases which follow be often convenient to employ a single term 

 to denote this phenomenon wherever and however occurring. 

 For this purpose the term Merism will be used. The introduc- 

 tion of a new term is, as a practice, hardly to be justified; but in 

 a case like the present, in which it is sought to associate divers 

 phenomena which are commonly treated as distinct, the employ - 

 "ment of a single word, though a new one, is the readiest way of 

 giving emphasis to the essential unity of the phenomena comprised. 



The existence of patterns in organisms is thus a central fact 

 of morphology, and their presence is one of the most familiar 

 characters of living things. Anyone who has ever collected 

 fossils, or indeed animals or plants of any kind, knows how in 

 hunting, the eye is caught by the formal regularity of an organized 

 being, which, contrasting with the irregularity of the ground, is 

 often the first indication of its presence. Though of course not 

 diagnostic of living things, the presence of patterns is one of their 

 most general characters. 



On examining more closely into the constitution of Repetitions, 

 they may be seen to occur in two ways ; first, by Differentiation 

 within the limits of a single cell, as in the Radiolaria, the sculpture 

 of egg-shells, nuclear spindles, &c, to take marked cases ; and 

 secondly, by, or in conjunction with, the process of Cell- Division. 

 The Symmetry which is found in the Serial Repetitions of Parts in 

 unicellular organisms does not in all probability differ essentially 

 from that which is produced by Cell-Division, for, though suffi- 

 ciently distinct in outward appearance, the two are almost cer- 

 tainly manifestations of the same power. 



