270 THE ZOOLOGIST. 



live. These are not stages in an upward road. I feel sure that 

 the process has been the other way, and think, moreover, that I 

 can see a reason for it. During the spring of 1918 I watched, 

 continuously and from day to day, the domestic economy of a 

 pair of Eed-throated Divers.* One of the two chicks died in the 

 way I have described, but, even after this, the surviving one, 

 that had now two parents to attend to it, was fed by them at 

 very long intervals. One must suppose that it got enough, but, 

 with anything like a struggle, two would be less likely to, so that, 

 as one robust chick is better than two weakly ones, it would be 

 for the benefit of the species to have this change brought about. 

 This would be all the more the case because, as here implied, it 

 is the habit of the parents, when there are two chicks, each to 

 take charge of one of them, and, as they then separate, this 

 would prevent either chick from benefiting at the expense of the 

 other and less hardy one, which would be the same principle, but 

 less effectively applied. It is indeed possible that the one chick, 

 in the instance observed by me, died because the sole parent in 

 whose charge it was did not sufficiently nourish it, but it did not 

 appear to me to be more neglectful in this respect than was the 

 other. On the other hand, the lesser activity of the chick that 

 succumbed — its more lethargic ways t — was noticeable from the 

 beginning, and seemed to be the governing factor. I cannot 

 myself think of a quite satisfactory explanation why the chicks 

 -of these Divers should be fed at such long intervals, but the 

 grown birds have themselves to cater for, as well as their young, 

 and the impartial law of natural selection will not more have 

 favoured the efforts of the bird to catch fish than of the fish to 

 •avoid being caught. It may be a hard struggle, and should the 

 food supply ever run short, a single year might see the death, 

 through starvation, of a large number of chicks. Obviously, 

 therefore, one chick that was tended by both its parents would 

 have double the chance of surviving that either of the two tended 

 by one of them only would have, whilst, at the same time, the 

 burden upon each of the parents would be only half as great. I 

 do not know whether, owing to long habit, become stereotyped, 

 the young of the Eed-throated Diver are fed on one kind of fish 



'■' See resume of my notes in ' Wild Life ' for 1914. 

 t See ante, p. 268. 



