Xajure of HKr.:F.DiTY. 321 



case of the Alpine plants above referred to, for example, leaves already 

 fully formed at the time of removal will not he modified by the 

 change of climate ; it is the new leaves, formed subsequently to the 

 removal, which will shew the adaptive modification. Organs when 

 once fullv formed tend to become fixed and incapable of responding 

 in this manner to changed conditions of life. 



Amongst adult animals, which have ceased to produce new 

 growth, we see few conspicuous examples of such adaptive response 

 to the stimulus of changed conditions. Tn some immature animals, 

 however, it may be observed in a very striking manner; as. for 

 example, in the case of tadpoles, which, when prevented from coming 

 to the surface of the water by means of wire netting, will continue 

 to develop as tadpoles instead of undergoing the normal meta- 

 morphosis into frogs. 



As part of the direct action of the environment we may also 

 convenientK include the Lamarckian principle of Use and Disuse as 

 affecting the <leveloj)ment of orgaris, for we must always remember 

 that the bodv to a large extent constitutes its own environment, 

 antl that alterations in the mode of action of the different organs 

 of the bodv mav produce corresponding effects upon the structure 

 of those organs. 



\\ is not necessarv for our argument to suppose that the 

 stimulus of changed conditions of life always produces adapin\' 

 response on the part of the organism, though there is abundant evi 

 dence to shew that this is frequently the case, as in the Alpine plants 

 and tadpoles referred to above, while those modifications which are 

 unsuited to (he environment will of course be weeded out by natural 

 selection. For suggestions as to the manner in which the action 

 of the environment mav produce adaptive modifications in the in- 

 dividual organism. 1 mav refer to the writings of Lamarck, Herbert 

 Spencer, and Cope. "In animals therefore, as in plants," sa\s 

 Spencer. " the external mechanical actions to be resisted are them 

 selves directlv instrumental in working, in the tissues they fall upon, 

 the changes which fit those tissues to meet them." 



("ope gives ver) conclusive examples in the ca.se of the formation 

 of the joints (jf the vertebrate skeleton, from which he draws the 

 following conclusions : — 



'■ First. Continued excessive friction removes o.sseous tissue 

 from the points of contact until complete adaption is accomplished 

 and the friction is reduced to a normal minimum. Then a normal 

 articular surface is produced. 



'' Second. Where the normal friction is wanting, and an in- 

 flammatory condition is maintained by a pulling stress on the invest- 

 ing s\novial meml)rane, excess of osseous deposit is produced. 



" Third. Stress on the articular ligaments and tendons stimu 

 lates osseous deposit at their insertions, which deposit mav be con 

 tinned into their substance. This is a pulling stress. 



These observations, therefore, show that osseous deposit is 

 produced bv different forms of mechanical stimulus." 



