282 CONCLUDING CHAPTER 



rounding medium giving rise to a discontinuous 

 change in structure. Such cases are, however, com- 

 paratively rare. Much more usually the changes in 

 external conditions are continuous, as changes of 

 altitude, moisture, or chemical composition of the soil, 

 and so on ; and the changes induced by them in the 

 plant are similarly of a continuous kind. 



In most animals changes in external circumstances 

 have a much smaller influence on the form and struc- 

 ture of the individual than is the case with plants. In 

 animals considerable modifications are, however, 

 brought about by exercise and the use of different 

 parts, as Lamarck long ago observed. But these 

 modifying factors usually affect all the members of a 

 single species in nearly the same manner. Neverthe- 

 less, some part of the differences between individuals in 

 respect of strength and of proportion, and possibly also 

 of stature, is undoubtedly associated with differences 

 of training and nutrition, as the example of the human 

 race is sufficient to show. Professor Cope has pointed 

 out how the proper development of such structures as 

 the joints of vertebrates depends to a very large 

 extent upon exercise ; and the effect of disuse may be 

 practically tested by anyone whom accident obliges 

 to keep a knee or other joint immovable for any length 

 of time. The so-called play in which the young of 

 many animals indulge for example, lambs and kittens 

 must have a great influence upon the perfection of 

 their locomotory functions. 



We can now see more clearly the reason for that 

 great instability of vegetative type which sessile 



