DISEASE IN ANIMALS AND IN MAN 253 



Disease more Frequent in Man than in Animals. — Diseases 

 occur among wild animals, but, so far as we can judge, they are 

 very rare. They are certainly rare when compared with the 

 frequent diseases of mankind. Why is this ? One reason, 

 probably, is that natural selection has a grip on wild life that 

 man has refused to allow it to have over him. Elimination is 

 keener and the wild race is healthier. Animals born diseased 

 are killed off before they can reproduce. To parasites they 

 adjust themselves, or become immune. Another reason is that 

 wild animals live " more natural " lives, and that the stimuli 

 provoking disease are therefore fewer. A third reason, perhaps, 

 is that man is relatively younger than most wild races, and, 

 therefore, with more idiosyncrasies. Fourthly, it seems that 

 where epidemics occur among wild animals, they are almost 

 invariably due to human interference. (See Ray Lankester's 

 Kingdom of Man, 1907, p. 32.) 



It should also be recognised that man has created around 

 himself a social heritage which often evolves quickly, hurrying 

 and pressing its creator, who cannot always keep pace with 

 it. This is a frequent condition of mental disorder. More 

 generally, we may venture to say that many human diseases, 

 especially of a nervous sort, seem in part due to the fact that 

 the germ-plasm is not varying quickly enough to keep pace with 

 the changes in environment — physical, biological, psychical, and 

 social. We try to adjust ourselves to these by a panoply of 

 modifications, and this business of adjustment is a strain 

 that provokes disease. 



As the physiological and the pathological are really but 

 two aspects of the general problem of vital activity, it is mainly 

 for practical reasons that we have ventured to devote a special 

 chapter to the facts of inheritance in connection with disease. 

 Apart from practical interests, it will be seen that, though the 

 available facts in regard to disease do not lead us to any 

 novel considerations which are not illustrated in normal cases, 



