214 DOCTRINE OF EVOLUTION 



and shape of the eye, it remains true that races do differ 

 intellectually, and that their differences are marks of 

 a mental evolution quite as definite as their physical 

 natural histories of change. 



In my own view the strongest and most impressive 

 evidence bearing upon the great problem before us is 

 provided by the series of transformations by which the 

 human intellect develops during an individual life. 

 Mind has an embryology no less significant than that 

 of the skull or of any other element of the body; and 

 its investigation leads to the evolutionary interpreta- 

 tion quite as surely as the study of the various grades 

 of adult psychology constituting the anatomical se- 

 quence, which we have reviewed previously. When in 

 the earlier part of the book we dealt with embryology 

 in general, we learned how the changes which take place 

 when an organism develops from an egg demonstrate 

 the actuality of true organic transformation without 

 the necessity of concluding or inferring that this process 

 might occur. It is not superfluous to insist again that 

 the essential fact in evolution is the alteration of one 

 organic characteristic into another type ; must we not 

 recognize at the very outset that mental transformation 

 is as real as physical development ? 



In the first instance we might concern ourselves with 

 the physical basis of mind and its history. In the 

 earliest stages of human embryology no nervous system 

 whatsoever is present, and it is unreasonable to suppose 

 that there is anything going on which corresponds to 

 human thought. A little later a cellular tube is estab- 



