312 MERISTIC VARIATION. [part i. 



of Variation with reference to this question is so intimately con- 

 nected with the subject of the variation of digits in general that 

 it cannot be considered apart. Other cases referring to repetitions 

 in the line of the axis of appendages will be taken in a subsequent 

 chapter. 



In studying numerical Variation in the digits of certain animals, 

 especially the Horse and the Pig, we shall meet with forms of 

 Variation which are peculiar to structures having a bilateral 

 symmetry. In examining the evidence as to Meristic Variation 

 of Bilateral Series further reference to these cases will have to 

 be made, but it appears simplest to describe the facts in the first 

 instance in connexion with the subject of digits. 



From the evidence as to Meristic Variation in digits I propose 

 to make a selection, taking certain groups of cases having a direct 

 and obvious bearing on the general problems of Variation. It 

 will be understood and should be explicitly stated that unless 

 the contrary is declared the principles of form which can be per- 

 ceived as operating in special cases are not of universal appli- 

 cation in the Variation of digits, but are enuntiated as applying 

 only to the special cases in which they are perceived. In the 

 human subject, for example, cases of polydactylism will be quoted 

 which when arranged together form a progressive series illus- 

 trating the establishment of a novel and curious Symmetry ; but 

 though these cases are valuable as illustrations of the way in 

 which the forces of Division and growth can dispose themselves 

 to produce a symmetrical result, yet it must always be borne in 

 mind that very many variations of the digits have been seen in 

 Man, whether consisting in increase in number of digits or in 

 decrease, of which the result is almost shapeless. The case of 

 polydactyle Cats is thus especially interesting from the fact that 

 in this animal the polydactyle condition, though differing in degree 

 of expression in various specimens, yet, in the greater number of 

 cases, occurs in ways which may be interpreted as modifications 

 of one plan, or rather of one plan for the hind foot and of another 

 for the fore foot. 



I arrange the evidence primarily according to the animal con- 

 cerned, Cat, Man and Apes, Equidae, Artiodactyles, &c. To these 

 are added a few facts as to digital variations in Birds, but from 

 the scantiness of the evidence and the difficulty of determining 

 the morphology of the parts I have not found it possible to give a i 

 profi table account of these phenomena in other vertebrates below 

 Mammalia. 



In most of the groups increase in number of digits may be 

 seen to occur in several distinct ways ; and, just as in the case 

 of teeth, mammae, &c, it is possible to recognize cases of division 

 of single members of series, and cases of addition to the series 



