TUBERCULOSIS AMONG ANIMALS 59 



has led to their being spoken of as " children's diseases." This 

 . lends further support to the theory of the gradual acquirement 

 of immunity by natural selection, for, as Dr. Archdall Reid 

 points out, " children represent a stage in the life history of the 

 race when the general evolution against disease was not so 

 advanced." 



The gradual increase of immunity in the series of animals 

 described above seems to afford a striking instance of pro- 

 gressive adaptation. It would be interesting, and would serve 

 as some test of the theory, to know how far the relationship 

 holds among animals other than those examined by the 

 Committee. We should, for example, expect horses and sheep 

 to resemble generally bovines and pigs ; while deer, foxes, and 

 squirrels should, like rabbits, be highly susceptible. Of particular 

 interest would be some experiments on voles and other forms 

 nearly allied to rats, but living in a different environment. 



It is difficult to regard the variation upon which natural 

 selection has acted in bringing about this evolution as other than 

 " continuous." The observation, therefore, has additional value 

 at a time when adherents of the advanced Mendelian school 

 are suggesting that discontinuous variations have alone played 

 a part in causing evolutionary changes. From the practical 

 point of view the lesson taught is that while we may endeavour 

 to maintain the health of our domesticated cattle by surrounding 

 them with sanitary conditions, every effort should be made to 

 discourage breeding from tainted individuals. It is only by- 

 assisting nature that we shall ever be able to call into existence 

 naturally resistant stocks. 



