132 PIONEERS OF EVOLUTION PART 



in my life have been more satisfactory to me that 

 we have never felt any jealousy towards each other, 

 though in one sense rivals. I believe I can say this 

 of myself with truth, and I am absolutely sure it is 

 true of you.' 



But on one question, round which discussion 

 still rages, the friends were poles asunder. There 

 had been correspondence between them as to 

 the bearing of the theory of natural selection on 

 man, and in April 1 869 Darwin wrote, ' As you 

 expected, I differ grievously from you, and I am 

 very sorry for it. I can see no necessity for call- 

 ing in an additional and proximate cause in re- 

 gard to man.' In the fifteenth chapter of his com- 

 prehensive book on Darwinism, Wallace admits 

 the action of natural selection in man's physical 

 structure. This structure classes him among the 

 vertebrates ; the mode of human suckling classes 

 him among the mammals ; his blood, his muscles, 

 and his nerves, the structure of his heart with its 

 veins and arteries, his lungs and his whole respiratory 

 and circulatory systems, all closely correspond to 

 those of other mammals, and are often almost 

 identical with them. He possesses the same number 

 of limbs, terminating in the same number of digits, 

 as belong fundamentally to the mammals. His 

 senses are identical with theirs, and his organs of 

 sense are the same in number and occupy the same 

 relative position. Every detail of structure which is 

 common to the Mammalia as a class is found also in 

 man, while he differs from them only in such ways 

 and degrees as the various species or groups of 



