i9oi- Moffat. — The Spring Rivalry of Birds* 153 



altoget1ier~spasmodic. A terrible storm, during the short time 

 of their passage, may destroy them in multitudes, just as a 

 terrible winter may wreak enormous havoc among our resident 

 birds at home. But these catastrophes do not occur with such 

 frequency as is needed to account for birds not increasing in 

 '* geometrical ratio," and when we look to more ordinary 

 checks on multiplication, while I admit to the full our very 

 meagre knowledge of what those checks are, I say we ought 

 not to accept an unverified assumption that they always work 

 by killing. 



I think that Darsvin, with all his clear-sightedness, did 

 assume this. He kept harping on the doctrine, which seemed 

 to him self-evident, that " more individuals are produced than 

 can possibly survive." In one passage of the Oidgin of 

 Species we are told that '' every being which during its 

 natural life produces several eggs or seeds, must suffer 

 destruction during some period of its life, and during some 

 season or occasional year ; otherwise, on the principle of 

 geometrical increase, its numbers would quickly become so 

 inordinately great that no country could support the product." 

 " A struggle for existence inevitably follows from the high 

 rate at which all organic beings tend to increase." It is true 

 that in another passage w^e read that the term " struggle for 

 existence " is used in a " large and metaphorical sense/' the 

 real object being ''not only the life of the individvial, but 

 success in leaving progeny." That distinction is important, 

 and it is one that Darwin never lost sight of as regards plants, 

 which he well knew might live for a long time unfertilised. 

 But it seems to me that it never occurred to him to give it its 

 due consideration as regards animals. He does not consider 

 the possibility of any appreciable number of animals living 

 the lives of old bachelors and old majds. In fact, he explicitly 

 states that there can, in the animal world, be "no prudential 

 restraint from marriage." This, I venture to claim, was too 

 summary a mode of dismissing the question. The main 

 object of my present paper is to show — or at least to make it 

 seem probable — that there are checks of a prudential kind on 

 the marriage of birds, and that these checks may be a very 

 important factor in keeping the number of birds absolutely 

 permanent. > ,•.- 



