336 POPULAR SCIENCE MONTHLY 



EXTEA DIGITS AND DIGITAL KEDUCTIONS 



Bv Dr. CHARLES W. PRENTISS 



UNIVERSITY OF WASHINGTON 



A LTHOUGH the mammalian extremities are nicely adapted by 

 -*--*- their structure to the functions they perform, the number of 

 digits frequently varies from the normal. Moreover, different degrees 

 of digital reduction may be observed in the extremities of animals whose 

 habits are apparently identical. It is generally recognized that the 

 digits of many mammals have been reduced to adapt the foot to rapid 

 locomotion, but the evidence is chiefly circumstantial. In the present 

 paper the writer will attempt to reconcile the various theories account- 

 ing for supernumerary digits, to call attention to certain evidences of 

 reversion which may be daily observed, and to point out some little 

 recognized factors concerned in the evolution of the mammalian foot. 

 We may assume that the primitive and typical mammalian foot was 

 pentadactyl, in spite of Bardeleben's contention that the progenitors 

 of the mammalia possessed not five but seven digits. Bardeleben's 

 assumption was based upon the observation that certain mammals, the 

 whale, for example, have more than five digits; that among five-toed 

 forms six and seven digits occasionally occur ; and that in many species 

 small cartilages are present on each side of the hand and foot. These 

 cartilages Bardeleben regards as digital rudiments, and the occurrence 

 of extra digits is explained by him as reversion, a ' turning back ' 

 through heredity, to ancestral conditions. Unfortunately, the facts do 

 not support this beautiful theory. Paleontology tells us that the fore- 

 runners of the mammalia possessed only five toes. Embryology has 

 shown that the sixth digit of the whale, and the cartilages which Bar- 

 deleben supposes to be digital rudiments, develop secondarily some 

 time after the typical five digits have appeared. Finally, observations 

 have proved that the extra digits which occur in polydactylism do not 

 develop from Bardeleben's ' digital rudiments,' but originate in an 

 entirely different manner. We may, therefore, assume that the primi- 

 tive mammalian foot was pentadactyl, and this being so, the occurrence 

 of six or seven digits on a foot normally five-toed can not be attributed 

 to reversion, unless we assume with Albrecht that it is reversion to the 

 many-rayed fins of the Elasmobranch fishes, an absurd supposition. 

 Such cases of polydactylism are, nevertheless, of frequent occurrence 

 on the appendages of man and the cat. They have been explained as 

 due to bifurcations or duplications of one of the typical five digits. 

 Dissections show that this is really the case, for, though the skeletal 



