162 THE PRINCIPLES OF HEREDITY 



will appear in subsequent generations. 1 The fact that the 

 sick birds were more or less sterile does not bear on the 

 point at issue. No doubt, when the birds are ill, the germ- 

 cells, like the rest of the cells, are incapable, in a greater or 

 lesser degree, of fulfilling their functions. It has long been 

 known that ill-health is a cause of sterility. But the state- 

 ment that the offspring were feeble is a very remarkable one. 

 It would seem, if Professor Ewart's conclusions be accepted 

 as embodying a general truth, that spermatozoa, which leave 

 the male during his illness, are so weakened that even within 

 the healthy female body, and after union with healthy ova, 

 they produce debilitated offspring. In other words, such 

 cells produce weakened cell-descendants. Neither in them- 

 selves nor in their descendants do they tend to recover. But 

 if these same sperms reside for a longer time within the 

 unhealthy body of the male, they can, in common with all 

 the rest of his cells, recover perfectly and produce normal 

 offspring when he recovers. So that the conclusion we are 

 driven to is this, that the only cells in the whole body in- 

 capable of recovery are those ripe germ-cells which happen 

 to pass to a healthy environment (the female body), and 

 which enter into union with healthy cells. Professor Ewart 

 himself believes that ripe germ-cells are specially liable to 

 alterations which change their hereditary tendencies. He 

 does not tell us what proportion of his Indian birds were ill, 

 but apparently all, or nearly all, were infected. One, at 

 least, was an old bird. It appears, then, that the disease is 

 widely prevalent among pigeons in India, and that immunity 

 is not easily acquired against it, any more than it is against 

 malaria. How, if the disease is very prevalent, if it is of 

 long duration, if infected birds tend to be sterile, if such 

 offspring as they produce are enfeebled, and if these en- 

 feebled offspring are exposed to the attacks of the parasite, 

 the species continues to exist in its native habitat is a mys- 

 tery. As a fact, however, of all Professor Ewart's observa- 

 tions only a single one lends support to his belief that off- 

 spring are enfeebled by parental disease. In this case two 

 Indian birds, of which the male was much diseased and the 

 female little affected, were mated. The union proved fertile, 

 but the offspring perished, a circumstance which is obviously 

 susceptible of several explanations more probable than the 

 one Professor Ewart has given. 



1 Their reappearance is rendered probable by the experiments of 

 Mendel (see notes to 104 and 129), Darwin (Animals and Plants, vol. 

 ii., p. 46), and other observers. 



