104 



ANIMAL MECHANISM. 



In all these modes of locomotion a force is expended which 

 impels in opposite directions two bodies more or less resisting ; 

 the one is the fulcrum, the other the weight to be displaced. 



Old writers called the force acting on the boat re-action — 

 they considered it as an effort emanating 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 each 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 movability, one of them will 

 be displaced more than the other. 



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

 motion ; it will be sufficient 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 

 principal types of terrestrial motion, that swimming corre- 

 sponds with the more habitual mode of aquatic locomotion, 

 and flight 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 ; flight is habitually performed in the air, and yet 

 certain birds take a decided* flight in the water. 



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

 particular type of locomotion, our embarrassment would be 

 as great as if we were classifying these movements. Some, 

 indeed, move with an equal facility on the earth, the water, 

 and in the air. We will not therefore attempt a strictly 

 methodical classification of the different modes of locomotion 

 of which we are about to take a rapid survey. 



Terrestrial locomotion furnishes two principal types : in one 

 the effort consists in pressing on the ground in the direction 



