MERISTIC VARIATIONS 149 



about a mean or average condition, are contrasted with larger 

 discontinuous variations, or mutations. 



Somatogenic variations, which affect the body of the 

 organism and are acquired in the life-time of the individual, are 

 contrasted with blastogenic or germinal variations, which arise as 

 a consequence of some modification in the germ cells. 



These three methods of classification obviously overlap one 

 another. Thus a meristic variation may be continuous or 

 discontinuous, somatogenic or blastogenic, and so on, but it will 

 be convenient to deal with each group separately. 



Meristic or Numerical Variations. In a very great number of 

 organisms certain parts are repeated with a greater or less 

 degree of regularity and constancy, and it is upon this repetition 

 that the symmetry of the organism very largely depends. An 

 ordinary star-fish, for example, is radially symmetrical, with five 

 similar arms or rays radiating from a common centre ; meristic 

 variation, however, not infrequently gives rise to six-rayed 

 individuals. The common jelly-fish, Aureliu aurita, again, usually 

 has four principal radii, but specimens are occasionally found with 

 two, three, or six. 



All vertebrate animals, on the other hand, and many inverte- 

 brates, are bilaterally symmetrical, and at the same time meta- 

 merically segmented, i.e. with repetition of similar parts in linear 

 series one behind the other. The vertebral column, for example, 

 is made up of a larger or smaller number of morphologically 

 equivalent units, the vertebrae, to which the ribs are attached in 

 linear series on each side, and the limbs are arranged in two pairs, 

 in each of which the same fundamental structure can be traced. 

 Many instances occur in different animals of variation in the 

 number of vertebrae, and in man the occasional occurrence of 

 thirteen ribs instead of the normal twelve on each side is well 

 known. Cases of polydactylism, or -the development of extra 

 digits on hand or foot, also come under the head of meristic 

 variation. 



In the vegetable kingdom, where the repetition of similar 

 parts, such as the leaflets of leaves, is even more conspicuous 

 than amongst animals, meristic variation is again a common 

 phenomenon. A clover leaf, for example, normally consists of 

 three leaflets, but the occasional discovery of a specimen with four 

 has been a source of satisfaction to superstitious folk from time 

 immemorial. We meet with similar variations in the number 



