CHAPTER XI 

 ON THE NATURE OF LIFE 



In our own time, as at the middle of the seventeenth century, 

 speculative biology has come to an impasse. Now, a word more 

 about the Cartesian physiology. Descartes, as we have seen,* 

 made a mechanical theory of the animal body, explaining all the 

 ordinary functioning — that is, the motions of the muscles and 

 fluids and the activities of the glands — by means of the physics 

 known to him. He saw very clearly that there was little 

 difference between the anatomical structure of man and that of 

 the other mammals, and so he was compelled to hold that in 

 both cases the animal body was an automatic machine, not 

 essentially different from those that had been constructed by 

 expert mechanicians, but, of course, immensely more complicated. 

 Nevertheless, there were profound differences between the 

 behaviour of man and that of the lower animals, and to account 

 .for this he supposed that the human body possessed a " sensitive 

 soul " which was not contained in the bodies of other animals. 



It would be difficult for a biologist to support this latter 

 assumption in these days, because we are now thoroughly con- 

 vinced that an evolutionary process has occurred, and that there 

 must, therefore, be absolute continuity between the human and 

 animal minds. We have also studied the psychology and 

 behaviour of the lower animals with a degree of care, and an 

 intuitive understanding, of which we can find no trace in the 

 biology of the Continental schools of the seventeenth century. 

 Further, the study of development shows that precisely the same 

 processes occur during the train of events along which the mature 

 animal body comes from the unfertilised egg, whether we in- 

 vestigate man, or the higher mammal, or the fowl, or the fish. 

 Therefore we cannot now place a " sensitive soul " in the human 

 body and deny its presence in that of the lower animal. 



In the seventeenth century, then, there was an absolute dis- 



' tinction between man and the lower animals. The former was 



dominated or controlled by the soul, which was an immaterial, 



* In Chapter IX. 

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