TIIE PAST EVOLUTION OF MAN 79 



regard this fact as a new " proof of the close blood- 

 relationship, in the literal sense of the word," between 

 man and the anthropoid ape. 1 



This most remarkable series of researches may 

 be supplemented by certain recent observations in 

 comparative pathology. It is as yet very far from 

 being precisely known why certain species or indi- 

 viduals are immune from certain diseases — i.e. from 

 the attacks of certain microbes — whilst others are 

 susceptible. One kind of sheep, for instance, is 

 entirely immune to the attacks of the anthrax 

 bacillus, whilst all other kinds are susceptible. 

 The cause of the difference is doubtless to be found 

 in the most intimate regions of the chemistry of 

 the body-cells. Now — in virtue, who can doubt, of 

 his ultimate relationship to other animals — man 

 shares his susceptibility to many diseases with 

 various members of what were once called the 

 " lower orders of creation." The resemblance between 

 his cell-chemistry and that of the cow, that of the 

 bird, and even that of the fish, is close enough to 

 permit of his susceptibility, in common with all of 

 these, to the attacks of the bacillus tuberculosis. 

 Some diseases, however, are peculiar to man, as 

 others are peculiar to other animal species : the 

 limited scope of these diseases doubtless being due 

 to the fact that the cell-chemistry of each species 

 varies, in some measure, from that of every other. 2 

 Man, for instance, suffers from measles and whooping- 

 cough. The lower animals, so far as we know, do 



1 See "The Evolution of Man" (English translation), by Ernest 

 Haeckel, vol. i. pp. 399-401. 



2 In the last resort, the difference between one species and 

 another is doubtless a matter of cell-chemistry. 



