2,3G THE INDUCTIONS OP BIOLOGY. 



stimuli; and since the functions of these subsidiary organs 

 are dependent on the functions of organs which supply them 

 with materials and stimuli; it follows that before any great 

 extra power of discharging its function can be gained by a 

 specially-exercised organ, a considerable extra power must 

 be gained by a series of immediately-subservient organs, and 

 some extra power by a secondary series of remotely-sub- 

 servient organs. Thus there are required numerous and 

 wide-spread modifications. Before the artery which feeds a 

 hard-worked muscle can permanently furnish a large addi- 

 tional quantity of blood, it must increase in diameter; and 

 that its increase of diameter may be of use, the main artery 

 from which it diverges must also be so far modified as to 

 bring this additional quantity of blood to the branch artery. 

 Similarly with the veins ; similarly with the structures which 

 remove waste-products; similarly with the nerves. And 

 when we ask what these subsidiary changes imply, we are 

 forced to conclude that there must be an analogous group of 

 more numerous changes ramifying throughout the system. 

 The growth of the arteries primarily and secondarily im- 

 plicated, cannot go to any extent without growth in the 

 minor blood-vessels on which their nutrition depends; while 

 their greater contractile power involves enlargement of the 

 nerves which excite them, and some modification of that part 

 of the spinal cord whence these nerves proceed. Thus, with- 

 out tracing the like remote alterations implied by extra 

 growth of the veins, lymphatics, glandular organs, and other 

 agencies, it is manifest that a large amount of rebuilding 

 must be done throughout the organism, before any organ of 

 importance can be permanently increased in size and power 

 to a great extent. Hence, though such extra growth in any 

 part as does not necessitate considerable changes throughout 

 the rest of the organism, may rapidly take place; a further 

 growth in this part, requiring a re-modelling of numerous 

 parts remotely and slightly affected, must take place but 

 slowly. 



