160 THE PRINCIPLES OF HEREDITY 



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 

 nearly all over, but in make not unlike the cross-bred turbit hen. 



267. " Some time before the second pair of eggs was laid, 

 the parasites had completely disappeared from the Indian 

 bird, and he looked as if he had quite recovered from his long 

 journey as well as from the fever. In due time a pair of 

 young were hatched from the second eggs, and as they 

 approached maturity it became more and more evident that 

 they would eventually present all the distinctive points of the 

 wild rock pigeon. The striking difference between the first 

 birds paired and the birds of the second nest might, however, 

 be due not to the malaria parasites but to the change of 

 habitat. 



268. " Against this view, however, is the fact that another 

 Indian bird infected to about the same extent as the mate of 

 the half-bred turbit counted for little when mated with a 

 second half-bred turbit; while two Indian birds in which 

 extremely few parasites were found at once produced blue- 

 rock-like birds when bred one with a fantail and the other 

 with a tumbler. 



269. " Another possible explanation of the difference 

 between the bird of the first and the birds of the second nest, is 

 that the germ-cells were for a time infected by the minute 

 protozoon Halteridium in very much the same way as the 

 germ- cells of ticks are infected by the parasite of Texas fever. 

 But of this there is no evidence, for even in the half-grown 

 birds hatched by the pure-bred malarious Indian rocks the 

 most careful examination failed to detect any parasites in 

 the blood. In all probability Halteridium can only be 

 conveyed from one pigeon to another by Culex or some other 

 gnat. 



