i MIND AS A FACTOR IN EVOLUTION 9 



world slight variations which turn out well are preserved. 

 The physical structure which made them possible is handed 

 on, and becomes the basis from which a fresh start can be 

 made. The structure of eye, wing, beak, and claw that 

 makes perfect the swoop and pounce of the falcon is the 

 result of countless ancestral variations in the course of which 

 the structure which made some slight approach to this 

 point of perfection prevailed over others, and prevailed 

 more surely the more nearly it reached its goal. 1 



Now Instinct, a term which has here been used in a 

 popular sense, is not when fully defined to be taken as 

 exclusive of Intelligence. On the contrary, as we shall 

 see later, the relations are subtle and closely interwoven. 

 But we may put the criticism in more radical fashion by 

 substituting for Instinct the conception of hereditary 

 physical structure acting in accordance with mechanical 

 laws. Every living being, it may be said, possesses such a 

 structure. It has grown up bit by bit under the con- 

 ditions of heredity and of the struggle for existence, every 

 variation that helped to keep the organism alive and 

 to perpetuate the stock being preserved, while every 

 adverse change is weeded out by the destruction of the 

 stock which it entails. This structure effects the corre- 

 lations which we find in the world of life. The spring of 

 the cat involves a cunning adjustment of numerous pro- 

 cesses, from the rise of the retinal excitement which we 

 call the sight of the mouse to the combination of muscular 

 contractions which lands the cat with her prey in her 

 claws. This adjustment subserves what, from the out- 

 sider's point of view, we call a purpose, the purpose of 



1 Later researches on heredity have thrown grave doubts on the 

 permanent value of small individual variations. Biology seems to be 

 reversing the old adage and proclaiming that in evolution natura nil 

 facit nisi per saltum. If this view is finally substantiated it must 

 profoundly affect the entire philosophy of development. Clearly it 

 postulates some originating factor of organic change the nature of which 

 is at present wholly unknown. Nevertheless the argument of the para- 

 graph stands, if it is clearly understood that " experience," i.e., realised 

 success in meeting the conditions of life, is not spoken of as the cause of 

 new experiments in behaviour but the condition governing the survival 

 of the individual making the experiment and of his line. We may still 

 regard a complex instinct like a complex structure as growing by stages, 

 though we may have to take each stage as a definite step, not as the 

 infinitesimal prolongation of a continuous curve. 



