94 MATERIALS FOR A MEMOIR ON 



The Significance of the Keel on the Metapodium. 



The presence of a ridge on the distal euds of the bones of the 

 tarsus and carpus, as well as on the first and second rows of pha- 

 langes, is indicative of a precision of motion in the joints of 

 which it constitutes a part. Such ridges are known as keels, and 

 are found in the feet of the ruminants, excepting the group of the 

 camels, and in the horse. The primitive types of mammalian life, 

 as has been pointed out by Professor E. D. Cope,* were without 

 the keel. It first appeared on the posterior surface of the articu- 

 lar surface, and subsequently upon its anterior surface as well. 

 These facts suggest the conclusion that the ridge was developed 

 for some exigencies of the foot while in backward strain, and in 

 the action of the transition from " on" to " oif," since the keel 

 first appears on the flexor side of the foot, and that, since in 

 extant animals the camel and its kin are without the keel, a 

 careful comparison of the motions in these creatures with those of 

 the other ruminants would be suggestive of the use of the keel, 

 and lead to some interesting differences between the gaits of 

 primitive and specialized types. 



After having made careful studies of the series of the Egyptian 

 camel (736), the Bactrian camel (737 to 742), and the guanaco 

 (743), I have been unable to find any features which correlate 

 with these anatomical peculiarities. 



Evolution of the Gait, 



The gait in its various expressions is based upon the correla- 

 tions between the fore and the hind limbs. Assuming that the 

 simplest movement is an alternate action of a single pair (syn- 

 chiry), the simplest gait is that in which this alternation occurs 

 in each pair independently. The gallop is thus the simplest gait, 

 since the fore pair and the hind pair are independent. But the 

 gallop depends upon a high momentum, and we must assume 

 that the primitive animals were slow walkers. When in a slow 

 rate the same independence is observed, — i.e., when the right fore 

 foot is moved forward at the same time as the right hind foot 

 (lateral heterochiry), — the two pair act rhythmically, and the feet 

 "keep step." This is seen in the rack, which thus becomes a 



* The Orisrin of the Fittest. 



