CHAPTER XXI 



THE MIND OF MAN 



The basis of man's mind The physical changes which have accom- 

 panied the evolution of memory Speech The substitution of ac- 

 quirements for instincts Sexual and parental love Modesty 

 Morality Comparison between physical and mental traits Mental 

 acquirements that are due to physical conditions The influence of 

 the environment Man the educable animal The opinions of 

 scientific men. 



438. AT the basis of all the physical structures of Man lie 

 his " inborn " physical characters. Some of his physical 

 characters develop to the fullest extent without the aid of 

 use-acquirements, which would be useless to them, and 

 which therefore they are incapable of making ; for example, 

 his hair which is never " used," his sexual organs which are 

 used to advantage only when fully developed, and his teeth 

 which are so constantly and equably used that Natural 

 Selection has been able to fix a right standard without an 

 unnecessary resort to use- acquirements. All his other 

 structures, for example his heart, his lungs, his limbs, his 

 brain, develop after birth chiefly if not exclusively as a re- 

 action to use. 1 Nutrition now supplies the material, not the 



1 We cannot say positively that the growth of these structures after 

 birth is due exclusively to use. The data for making a positive state- 

 ment are not available. But we know that a muscle completely 

 paralyzed tends to atrophy completely, and it is reasonable to suppose 

 that, if a muscle owes its preservation to use, it must owe its growth to 

 still more use. The paralyzed limbs of children remain diminutive ; 

 but I am not aware that anything has been published that demonstrates 

 whether or not a paralyzed limb on which no strain is placed increases 

 at all in size. The effect of use on the heart may be estimated by its 

 enormous hypertrophy in diseased conditions that place it under great 

 strain. The training of athletes renders their lungs very capacious. 

 It is hardly possible to doubt that man's brain owes its development to 

 use as much as any other structure. It is correlated to his mind, which 

 certainly grows under the influence of experience. The cerebral hemi- 

 spheres especially, the organ of his memory and all that flows from 

 memory, must depend on use for their increase after birth. If the 



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