170 INTELLIGENT BEHAVIOUR 
between them. We have now to consider the nature of this 
interaction, and to this end we must indicate the relation of 
acquired modifications to the hereditary groundwork of the 
animal constitution. 
The basal fact is, that the bodily tissues are subject toa 
certain amount of structural change during the course of 
individual life in accordance with the amount of functional 
strain put upon them. The labourer’s thickened skin, the 
enlarged and strengthened muscles of the athlete, the juggler’s 
acquired suppleness are familiar cases. Less familiar instances 
are afforded under abnormal conditions. Should one kidney 
from any cause be slowly destroyed, the other will slowly 
enlarge to carry on the increased work of elimination of waste 
products; when the larger shin bone of a dog has been 
removed after injury, the smaller bone becomes thickened to 
bear the added strain; new joint surfaces are sometimes 
formed where bones have been broken and the natural joints 
injured. 
One may say that the normal development of any structure 
depends upon a due amount of use. But, since in the course 
of strenuous life any organ is from time to time subject to an 
abnormal amount of strain, it must be fitted to respond to a 
super-normal call on its strength and functional activity. 
Were the heart and the lungs, for example, unable to meet the 
greatly added drain on their energies, due to unwonted and 
severe exertion, collapse, perhaps death, would ensue if such 
exertion were imperatively demanded under special circum- 
stances. And it is clear that many wild animals must be not 
infrequently placed in such circumstances as will subject their 
muscular structures and the functional activity of their organs 
of ‘circulation and respiration to a strain nearly up to their 
extreme limits of endurance. The carnivorous hunter would 
often fail to secure his prey if his organization were unequal to 
a hard and prolonged chase; the hunted prey would not 
survive to procreate his kind if he fell a victim to the first 
pursuer through inability to stand the exertion necessary to 
enable him to make good his escape. It is thus, we may 
