XV.] GEOV/TH OF ORGANS WITH EXERGISK 185 



by the direct action of the pressure of the enlarged muscles, 

 modifying the form and position of the other muscles and 

 of the bones — partly also, no doubt, by the increased nutri- 

 tion demanded by the enlarged muscles diminishing the 

 supply of nutrition to the other parts of the body, and so 

 compelling a diuiinution of their size. It is also to be 

 observed that, besides these secondary changes, as they 

 may be called, it is quite possible for change of habit, as 

 the result of new circumstances, to take place primarily in 

 two or more directions at once. Thus, for instance, the 

 necessity of pursuing a new kind of prey, or of pursuing 

 the same kind of prey in a different manner, as will occur 

 when forests are destroyed, may have the effect of causing 

 an animal to improve both in keenness of sight and in Improve- 

 fleetness. This appears to be not only a possible, but a gig^t and 

 probable case : keenness of sight and fleetness are often Aeetuess. 

 united in animals, as, for example, in birds of prey. 



In this chapter I have considered the laws of the forma- 

 tion and perjjetuation of habit ; in the next I shall have to 

 consider the laws of its variation. 



XOTE. 



GROWTH OF ORGANS WITH EXERCISE. 



It appears uncertain whether the increase in size of organs that Why do 



are much exercised can he accounted for by any physical cause ; "'S^"** . 



J J r J } grow with 



or whether, like the law of habit, it is an ultimate law of life, exercise ? 



and as such inexplicable. Herbert Spencer has made a most ^'=i'"<^" 

 ■^ _ ^ Spencers 



elaborate and ingenious attempt to prove that it is entirely due theory. 

 to the increased flow of blood that always takes place to and 

 through an organ in activity.^ He makes out an exceedingly 

 strong argument for beUeving that the deposit of woody sub- 

 stance in the vascular tissue of plants, which is the process by 

 ■which woody iibre appears to be formed, is originally due to the Woody 

 accelerated flow of the sap in the vessels near the surface of "'''^■ 

 ^ Principles of Biology, Part V. Chaps, iv., vii., and viii. 



