82 VARIABILITY 



" 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 of the second nest might, however, be due, 

 not to the malaria parasities, but to the change of habitat. 



"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. 



"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. 



" These results from pigeons suffering from malaria seem 

 to indicate that the germ-cells are liable to be influenced by 

 fevers and other forms of disease that for the time being diminish 

 the vitality of the parents. Further experiments may show that 

 the germ-cells are influenced in different ways by different 

 diseases. 



" Sometimes the germ-cells suffer from the direct action of 

 their immediate environment, from disturbance in or around the 

 germ glands. If, for example, inflammation by the ducts or other 

 channels reaches the germ-glands, the vitality of the germ-cells 

 may be considerably diminished ; if serious or prolonged, the 

 germ-cells may be as effectively sterilized as are the bacteria of 

 milk by boiling." 



133. Obviously Professor Ewart assumes that if offspring alter 

 in consequence of an alteration in the gerrn-^//, the change is 

 necessarily a variation. His experiments on fertilization are 

 interesting. He found " that when a well-matured rabbit doe is 



