MODES OF PROGRESSION. 95 



tacles as oars, (Fig. 47 ;) and some star-fishes (Comatula, 

 Euryale) use their arms with great adroitness, (Fig. 151.) 

 Finally, there are some insects which have their limbs con- 

 structed for running on the surface of water, as the water- 

 spiders, (Ranatra, Hydrometra.) 



Fig. 47. 



198. A large number of animals have the faculty of mov- 

 ing both in the air and on land, as is the case with most birds, 

 and a great proportion of insects. Others move with equal 

 facility, and by the same members, on land and in water, as 

 some of the aquatic birds and most of the reptiles, which latter 

 have even received the name Amphibia, on this account. 

 There are some which both walk, fly, and swim, as the ducks 

 and water-hens ; but they do not excel in either mode of 

 progression. 



199. However different the movements and offices per- 

 formed by the limbs may appear to us, according to the ele- 

 ment in which they act, we see that they are none the less 

 the effect of the same mechanism. The contraction of the 

 same set of muscles causes the leg of the stag to bend for 

 leaping, the wing of the bird to flap in the air, the arm of 

 the mole to excavate the earth, and the fin of the whale to 

 strike the water. 



