INFLUENCE OF INTELLIGENCE ON INSTINCT 169 
of this interpretation ; there is, moreover, little or no evidence 
of the assumed transmission to offspring of any acquired 
modifications of structure or behaviour. We have, therefore, 
been led to infer that instinctive behaviour has been evolved 
through the selection of adaptive variations of germinal origin, 
the influence of intelligence being restricted :to the fosterage 
of co-incident variations, that is to say, of those congenital 
variations which coincide in direction with the acquired 
modifications of behaviour due to intelligence. It is clear 
that on this interpretation the influence of intelligence on 
instinct is more indirect and less simple than that implied by 
the Lamarckian hypothesis. Intelligence and instinct are in 
large degree independent, though there is continual interaction 
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 to a 
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 jugeler’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 
