ANIMAL BEHAVIOR. 325 



(1) Disposition to Remain over the Eggs. - - The disposition 

 to remain over the eggs is certainly very old, and is probably 

 bound up with the physiological necessity for rest after a 

 series of activities tending to exhaust the whole system. If 

 this suggestion seems far-fetched, when thinking of birds, it 

 will seem less so as we go back to simpler conditions, as we 

 find them among some of the lower invertebrate forms, which 

 are relatively very inactive and predisposed to remain quiet 

 until impelled by hunger to move. Here we find animals 

 remaining over their eggs, and thus shielding them from 

 harm, from sheer inability or indisposition to move. That 

 is the case with certain molluscs (Crepidula), the habits and 

 development of which have been recently studied by Pro- 

 fessor Conklin. 1 Here full protection to offspring is afforded 

 without any exertion on the part of the parent, in a strictly 

 passive way that excludes even any instinctive care. In Clep- 

 sine there is a manifest unwillingness to leave the eggs, show- 

 ing that the disposition to remain over them is instinctive. If 

 we start with forms of similar sedentary mode of life, it is 

 easy to see that remaining over the eggs would be the most 

 likely thing to happen, even if no instinctive regard for them 

 existed. The protection afforded would, however, be quite 

 sufficient to insure the development of the instinct, natural 

 selection favoring those individuals which kept their position 

 unchanged long enough for the eggs to hatch. 



(2) Disposition to Resist Enemies. -- The disposition to keep 

 intruders from the vicinity of the nest I have spoken of 

 as an element of the instinct of incubation. At first sight 

 it seems to be inseparably connected with the act of covering 

 the eggs, but there are good reasons for regarding it as a 

 distinct element of behavior. In birds this element manifests 

 itself before the eggs are laid, and even before the nest is 

 built ; and in the lower animals the disposition to cover the 

 eggs is not always accompanied by an aggressive attitude. 

 This attitude is one of many forms and degrees. A mild 

 self-defensive state, in which the animal merely strives to hold 

 its position without trying to rout intruders, would perhaps 



1 Journ. of Morph., vol. xiii, No. I, 1897. 



