SIGNIFICANCE OF PREHALLUX AND PREPOLLEX. 35 



primitive hexadactyly or heptadactyly within known geological 

 times, it does not appear tenable to regard either of these 

 bones as accessory digits. Again, there is not a tendency for 

 the adcUtion of digits ; on the contrary, there is a pronounced 

 movement towards reduction, and Flower (1885, p. 283) has 

 summarised the manner of reduction as follows : — When one 

 digit is lost, it is usually the first, then follows the fifth ; the 

 third is always retained though either two or four or both of 

 these digits may be absent. 



. Graham Kerr (1919, p. 453), in commenting upon the 

 cheiropterygium, considers that there is greater anatomical 

 efhciency in the possession of a central digit supported on 

 each side by another digit ; and possibly the presence of an 

 additional digit outside these again is a further advantage. 

 This then implies that the third digit is the most essential 

 of the series, a conclusion quite in accord with Flower's Law 

 of Digital Reduction. 



As nature seems so insistent upon the expression of 

 symmetry, to obtain this state of affairs with a six-digital 

 manus or pes it would necessitate shifting the axis of symmetry 

 from the third digit to the interval between the third and 

 fourth. This may appear to be the case in regard to the 

 Artiodactyle Ungulates (e.g. pig), but when the carpus and 

 tarsus are considered it will be seen that the symmetry of 

 the appendage is fundamentally of the pentadactyl type, 

 and that the shifting of the symmetry of the digits ta the 

 interval between the third and fourth is quite secondary, 

 and only applies to the digits themselves. 



With reference to the Anuran prehallux, Gadow (1901, 

 p. 20) states that there are " five toes and the rudiment of a 

 sixth digit, the so-called prehallux, which consists of two to 

 four pieces, including the one representing the metatarsal. 

 This prehallux as a vestige of a once better developed digit 

 is exactly like the elements on the radial side of the wrist 

 which, we are certain, are the elements of a once complete 

 finger, the pollex. The only weighty difficulty against its 

 interpretation as a prehallux lies in the fact that hitherto no 

 six-toed Stegocephah have been found, but the fact that none 

 are known with more than four fingers could be used as an 

 argument against there being a pollex in recent Anura with 

 just as good reason." 



