THE BIOLOGICAL EVIDENCE 81 



the calf, becomes cow-pox ; returned to man, it is that very different 

 and much milder complaint, vaccinia. Rabies, passed through a 

 succession of monkeys, grows milder in type for man ; passed 

 through dogs, more virulent ; through rabbits, still more virulent. 

 When diseases are passed from one species of animal to another, 

 they are generally comparatively mild for the second species at 

 first, but tend to become more virulent to it the longer they afflict 

 it, till a maximum of virulence is reached. Therefore animals that 

 are affected by microbes that have inhabited their own, or a nearly 

 allied species, usually suffer more severely than when the microbes 

 are derived from a more alien source. The organisms of diphtheria 

 and tuberculosis may be gradually ' attenuated ' by cultivation in 

 artificial media, those of anthrax quite quickly by heat. Many 

 more instances of the same nature have been recorded. 



131. Medical men have published voluminous statistics proving 

 that diseased and intemperate people often have children degene~ 

 rate or defective physically or mentally, and that slum-bred 

 children are, on the average, inferior in physique to the offspring 

 of rural folk ; and they have assumed that these filial defects are 

 innate and due to the parental ill-health. 



132. In his Presidential Address to the Zoological Section of 

 the British Association, 1 Professor J. Cossar Ewart maintained 

 that " there is a considerable amount of evidence in support of the 

 view that changes in any part of the body or soma which affect the 

 general welfare influence the germ-cells." 



" It may first be asked," he added, " Does disease, in as far as it 

 reduces the general vigour or interferes with the nutrition of the 

 germ-cells, act as a cause of variation ? I recently received a 

 number of blue-rock pigeons from India infected with a blood 

 parasite (Halteridium), not unlike the organism now so generally 

 associated with malaria. In some pigeons the parasites were very 

 few in number, in others they were extremely numerous. The 

 eggs of a pair of these Indian birds with numerous parasites in the 

 blood proved infertile. Eggs of a hen-bird with numerous parasites 

 fertilized by a male with few parasites proved fertile, but the young 

 died before ready to leave the nest. An old Indian bird, however, 

 with comparatively few parasites, mated with a half-bred English 

 turbit, produced a single bird. The half-bred turbit has reddish 

 wings and shoulders, but is otherwise white. The young bird by 

 the Indian blue-rock is of a reddish colour all over, but in make 

 not unlike the cross-bred turbit hen. 



1 See Nature, vol. Ixiv., 1901. 



