101- ANIMAL Mr.rllANISM. 



In all those modes of locomotion a force is expended which 

 impels in opposite directions two bodies more or less resisting ; 

 tin- one is tin. 1 fulcrum, the other the weight to be displaced. 



Old writers called the force acting on the boat n'-uctiun 

 tln-y considered it as an effort emanating 1 from the soil, the 

 water, or any resistance whatever to which the, effort of the 

 rowers was applied. "We can now understand clearly that all 

 the motive force is derived from the boatman. This force can 

 have as its result, either the repulsion of two points to which 

 it is applied, or their approach to eacli other. In these two 

 cases one of the points may be fixed, it is then the other which 

 will be displaced ; or the two points may be movable, and 

 then, according to their unequal movubility, one of them will 

 be displaced more than the other. 



This geteral principle can be applied to all cases of loco- 

 motion; it will be Millie lent for us to notice that which is 

 essential in all the types which we shall consider. 



The most natural classification seems to be that which is 

 based on the nature of the point of resistance ; accordingly, we 

 may distinguish three principal forms of locomotion terres- 

 trial, aquatic, or aerial. But in each of these forms, what a 

 variety of mechanism we shall meet with ! 



If it be true that walking and creeping are the two 

 piincipal types of terrestrial motion, that swimming corre- 

 sponds with the more habitual mode of aquatic locomotion, 

 and fl'ght with aerial locomotion, it is not less true that in 

 certain media many kinds of locomotion are employed. Thus, 

 walking and creeping are used both on the earth and in the 

 water; iliglit is habitually performed in the air, and yet 

 certain birds take a decided Might in the water. 



In fact, if we were compelled to assign to every animal its 

 particular type of locomotion, our embarrassment would he 

 :<-.it as it' we -\\ere classifying tlieso movements. Some, 

 imlerd, move \\ith an Mpial facility on the earth, the water, 

 and in the air. "We will not then-fore attempt a strictly 

 methodical classification of the diHnvnt. modes of locomotion 

 of which wo are about to take a rapid survey. 



/. i'1'ixtriiil Inriiiiintinn furnishes two principal tvpes: in one 

 tin- dl'ort consists in pressing on the ground in the direction 



