ANIMAL LOCOMOTION. 95 



practicable gait for a primitive creature. The only objection to 

 this conclusion is the fact that in the giraffe, which is a specialized 

 and aberrant form, — and the legs are remarkable for their length, 

 — we have the best example of a natural pace. 



The simplest forms of moving appendages are seen in the 

 Nerita, one of the fresh-water worms, in which the bristle-like 

 rods are moved in all directions without concert. In other forms, 

 as in the sea-hare (Aphrodite), the bristles move in numerous 

 lines extending alongside of the body, but each line in turn being 

 independent of those adjacent to it. The next arrangement ob- 

 tains in myriapods, whose feet, while in pairs, are seen to move 

 as the horse's in pacing, — i.e., each row acting as a whole, and the 

 two rows alternating. In the lower crustaceans the same method 

 obtains. In the higher crustaceans, as in the crab, the two parts 

 of a single pair may act together. 



It stands to reason that when to one side of the body a re- 

 inforcement is received from a limb of the opposed side and of a 

 different pair advancing to the median line, a relatively high 

 degree of development has been attained. In a phrase, in animals 

 with highly developed central nervous systems the bilateral co- 

 ordinations become as practicable as the unilateral. 



The question of the manner of using the feet in progression is 

 also likely to be a subordinate one to the question of the size and 

 the bulk of the body. In short-legged, heavy animals of slow 

 movement, the support of the centre of the heavy trunk by the 

 diagonal use of a fore and a hind limb was probably early obtained. 

 Nevertheless the Echidna, a good example of an animal possessing 

 a large, heavy body and short limbs, walked by diagonal hetero- 

 chiry.* The feet, however, did not approach each other towards 

 the centre of the trunk, as is the case in the walk and the trot of 

 the horse, but kept to the side of the body. 



In long, slender natatorial forms, which would occasionally 

 slide among the ooze of a river-bed, — such an animal, for exam- 

 pie, as the Potomogale, — doubtless specialization can be attained 

 in the line of development of many of the gaits.f We have 



* This was observed in the specimen at the Zoological Garden, Philadelphia. 



f The crocodile of the Jumna, according to Hornaday (" Three Years in 

 the Jungle," 1885, p. 55), can stand upon the legs in the same manner as a 

 terrestrial quadruped. 



