238 CHILDHOOD OF ANIMALS 



between organism and environment is complete. If the proper 

 environment does not present itself, the instinct cannot come into 

 operation, and if it be necessary to the life of the animal, the animal 

 dies. No doubt instincts vary, like every part of the animal organ- 

 ism, and in course of time, by a continuous rejection of the less 

 suitable variations and a continuous preference of the more suitable 

 variations, an instinct might change. But so far as the individual 

 is concerned, the instinct is fixed. 



The operation of an instinct requires, in proportion to its com- 

 plexity, a certain complexity of structure, and unti the latter has 

 been attained it cannot take place. On the other hand, it does 

 not require practice, and there is no reason why animals that rely 

 upon instincts should have their period of youth longer than the 

 time required for bodily growth and development. In the verte- 

 brates, however, and especially and increasingly so in vertebrates 

 with high brain development, the rigid instincts are being broken 

 down and replaced by actions controlled by experience and by 

 memory, and so fitting more varied circumstances and more varied 

 environment. The period of youth is prolonged to afford time for 

 this. The animals are protected and cared for by their parents, 

 and allowed a space in which the burden of life does not press 

 heavily upon them, and in this time they have to educate their 

 instincts, destroy their rigidity, allow them to be controlled by the 

 stored-up results of experiment. The purpose of youth is to give 

 time for this, and therefore those animals which are most intelligent, 

 which have the most complex brains, have the longest period of 

 youth. 



