112 THE MECHANICAL FUNCTIONS. 



in many of the inferior ranks, we find all these objects re- 

 nounced for the more certain advantage of security, which 

 the softer texture of the organs renders one of paramount 

 importance. That construction of limbs which favours cer- 

 tain movements will necessarily interfere with the ready 

 performance of others, and must preclude the development 

 of the organs which would be necessary for facilitating them. 

 Different kinds of prey require dexterity in particular ac- 

 tions for their pursuit and seizure. The animal is, in one 

 case, formed for climbing trees; in another, for burrowing 

 in the earth: in a third, for perforating wood. Some are 

 provided with organs for penetrating into the bodies of other 

 animals; others with the means of insnaring their captives; 

 while others, again, instil into the veins of their victims a 

 deadly poison. Hence it is necessary, in studying the or- 

 ganization of animals, to bestow particular attention on the 

 habits and mode of life for which each respective tribe and 

 species has been destined. 



In the examination of the mechanical functions which will 

 form the first part of this treatise, I shall keep in view, as 

 the leading object of inquiry, the faculty oi p7'0gressive ino- 

 tion, noticing its different degrees of perfection as we fol- 

 low the ascending series of animals; but adverting, also, oc- 

 casionally, to the other topics which belong to this class of 

 functions. 



It may be observed in general, that the mechanical con- 

 struction of animals which constantly inhabit a watery ele- 

 ment is more simple than the construction of those which live 

 on land, and are encompassed by a lighter medium. Dif- 

 fering but little in their specific gravity from the fluid in 

 which they are immersed, aquatic animals are necessarily 

 supported, on all sides, by a powerful hydrostatic pressure, 

 which nearly balances the force of gravity, and counter- 

 acts the tendency of their bodies to descend in the fluid. 

 Many of the obstacles to progressive motion are thus re- 

 moved; and there is no necessity for the compactness of 

 frame, and the rigidity and cohesion of substance which are 

 required in terrestrial animals. 



