316 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 
