PRESSURE OF RESISTANCE. 325 



using them ; nay, the whole form of the body may be thus 

 determined, as in the turnspit, a dog with a propensity for 

 digging, or in the cylindrical form of the boring beetles of the 

 genus Bostrychus. It is clear that these adaptations of the 

 organisation of certain animals to the resistance offered by the 

 conditions under which they live must be a powerful means of 

 selection by which all the individuals which are not strong 

 enough to overcome the obstacles opposed to them must be 

 eliminated. And since any variation in the aggregate condi- 

 tions of the soi 1 , wood, or stone, in which such creatures live, 

 must be either very insignificant or perhaps wholly absent, 

 the point of ' stable equilibrium ' must soon be reached between 

 the strength of the digging organs on one side and that of the 

 obstacle to be overcome on the other ; the variations which easily 

 may occur in other surrounding conditions as in the tempei-a- 

 ture, moisture of the atmosphere, salt constituents of the water, 

 nutriment, <tc. can in such cases but very rarely be efficient 

 causes in effecting any modification in animals exposed to the 

 former influence. Nevertheless, such modifications of the 

 organs of many animals as were dependent on the aggregate 

 conditions of solid bodies must have occurred in the course of 

 their phyletic or generic development ; for, if we are to suppose 

 that boring or burrowing animals descended originally from 

 such as did not bore or burrow, in the process of modifying the 

 organs adapted for motion above ground into such as were 

 fitted for subterranean progress, organs which were originally 

 destined and contrived for walking, running, or swimming, must 

 have abandoned these functions to assume new ones, and thus 

 have been so far modified in structure that they could be used 

 in the best way for the purpose. But. we know nothing as to 

 how such alterations in the habits of certain animals urged 

 by some inward prompting may have been able to occasion 

 these modifications in the structural characters of the organs in 

 question. It is well known that a determined mode of motion, 

 or on the other hand the strength of the obstacle to be con- 

 quered, may have a certain effect on the strength of the muscles 

 called into play, and, through them, on the power of resistance 

 in the fulcrum joints of the bones, and ultimately on the 



