34 



VARIATION 



SECTION I SYMMETRY 



The central thought in all meristic studies is symmetry, by 

 which is meant that opposite sides of an organism possess parts 

 that are either identical or at least similar. Thus the petals upon 

 one side of a flower are in most cases like those upon the oppo- 

 site side, and the blossom is made up of a number of similar parts 

 very much alike and several times repeated. 1 



Among higher animals, however, opposite sides are similar but 

 not identical, and here arises the distinction between radial sym- 

 metry and bilateral symmetry. 



Radial symmetry and radial series. By this is meant that kind 

 of pattern in which the separate parts are identical and each part 

 is capable of replacing any other in the series. Common examples 

 are the petals of most flowers (leguminous and the like excepted), 

 the leaves and lateral shoots of plants, the capsules of many seeds, 

 such as the apple, orange, etc., the rows of corn upon the cob, 

 the parts of the sea urchin, the starfish, and the Radiolaria 

 generally. 



In all cases of this sort the individual parts could each replace 

 any other part of the series. The pattern is therefore spoken of 

 as one of radial symmetry, and the parts as members of a radial 

 series. 



Bilateral symmetry. Among higher animals a different sym- 

 metry is observable. While each side has its counterpart upon 

 the other, yet there is a distinction between the right and the 

 left sides and between the dorsal and the ventral surfaces. In 

 such cases the parts while similar are not alike, nor could they 

 replace each other. 



1 Symmetry is well-nigh universal. All organisms arise through cell division in 

 one or more planes, and some degree of symmetry is to be expected from the 

 manner in which growth takes place. 



But symmetry is not confined to multicellular structures. Appendages consist- 

 ing of single cells are frequently symmetrically placed, and many organisms, which 

 are single celled and therefore microscopic, as diatoms, secrete a skeleton with 

 regular markings as symmetrical as hoarfrost and quite as beautiful. 



All this is curious rather than valuable to the student of thremmatology who 

 is interested in multicellular beings ; yet it all throws light on the method of life, 

 and we are able to lay down the principle that symmetry is not only the natural 

 corollary of development by cell division, but that it is also a general principle in 

 living matter. 



