GENETIC STANDPOINT IN THE STUDY OF INSTINCT. 89 



animal merely strives to hold its position without trying to rout intruders, would perhaps 

 be the first stage of development. In some of the lower vertebrates the attitude remains 

 defensive and is aggressive only in a very low degree, while in others pugnacity is more or 

 less strongly manifested. Among fishes the little stickleback is especially noted for its 

 fiery pugnacity, which seems to develop suddenly and simultaneously with the appearance 

 of the dark color of the male at the spawning season. 



In pigeons, as in many other birds, this disposition shows itself as soon as a place for a 

 nest is found. While showing a passionate fondness for each other, both male and female 

 become very quarrelsome towards their neighbors. The white-winged pigeon (Melopelia 

 leucoptera) of the West Indies and the southern border of the United States is one of the 

 most interesting pigeons I have observed in this respect. At the approach of an intruder 

 the birds show their displeasure in both tone and behavior. The tail is jerked up and down 

 spitefully, the feathers of the back are raised as a threatening dog "bristles up," the neck 

 is shortened, drawing the head somewhat below the level of the raised feathers, and the 

 whole figure and action are as fierce as the bird can make them. To the fierce look, the 

 erect feathers, the ill-tempered jerks of the tail, is added a decidedly spiteful note of warning. 

 If these manifestations are not sufficient, the birds jump toward the offender, and if that 

 fails to cause retreat, wings are raised and the matter settled by vigorous blows. 



The pugnacious mood is periodical, recurring with each reproductive cycle and subsiding 

 like a fever when its course is run. The birds behave as if from intelligent motive, but every 

 need is anticipated blindly; for the young pair, without experience, example, or tradition, 

 behave like the parents. 



It seems to me that this mood or disposition, although in some ways appearing to be 

 independent of the disposition to cover the eggs, can best be understood as having developed 

 in connection with the latter. It has primarily the same meaning protection to the eggs 

 but the safety of the eggs and young depends upon the safety of the nest, and this accounts 

 for the extension of its period to cover all three stages building, sitting, and rearing. 



(3) Periodicity. The periodicity of the disposition to sit coincides in the main with 

 that of the recuperative stage. Its length, however, at least in birds, is nicely correlated 

 with, though not exactly coinciding with, the time required for hatching. It may exceed 

 or fall short of the time between laying and hatching. The wild passenger-pigeon (Ecto- 

 pistes) begins to incubate a day or two in advance of laying, and the male takes his turn 

 on the nest just as if the eggs were already there. In the common pigeon the sitting usually 

 begins with the first egg, but the birds do not sit steadily or closely until the second egg is 

 laid. The birds do not, in fact, really sit on the first egg, but merely stand over it, stooping 

 just enough to touch the egg with the feathers. This peculiarity has an advantage in that 

 the development of the first egg is delayed so that both eggs may hatch more nearly together. 

 The bird acts just as blindly to this advantage as Ectopistes does to the mistake of sitting 

 before an egg is laid. Ectopistes is very accurate in closing the period, for if the egg fails 

 to hatch within 12 to 20 hours of its normal time, it is deserted, and that too if, as may 

 sometimes happen, the egg contains a perfect young, about ready to hatch. 



Pigeons, like fowls, will often sit on empty nests, filling up the period prescribed in 

 instinct, leaving the nest only as the impulse to sit runs down. It happens not infrequently 

 that pigeons will go right on with the regular sequence of activities, even though nature 

 fails in the most important stage. Mating is followed by nest-making, and at the appointed 

 time the bird goes to the nest to lay, and after going through the usual preliminaries brings 

 forth no egg. But the impulse to sit comes on as if everything in the normal course had 

 been fulfilled, and the bird incubates the empty nest and exchanges with her mate as 

 punctiliously as if she actually expected to hatch something out of nothing. This may 

 happen in any species under the most favorable conditions. It is possible by giving an 

 abundance of rich food to wind up the instinctive machinery more rapidly than would 



