Laws and Hypotheses for Behavior 277 



incidental protection to offspring. That meaning is always 

 manifest, no less in worms, molluscs, Crustacea, spiders 

 and insects, than in fishes, amphibia, reptiles and birds. 

 The instinct makes no distinction between eggs and young, 

 and that is true all along the line up to birds, which extend 

 the same blind instinct to one as to the other. 



3 . Essential Elements of the Instinct. Every essential 

 element in the instinct of incubation was present long 

 before the birds and eggs arrived. These elements are: 

 (i) the disposition to remain with or over the eggs ; (2) the 

 disposition to resist and drive away enemies ; and (3) perio- 

 dicity. The birds brought all these elements along in 

 their congenital equipment, and added a few minor adap- 

 tations, such as cutting the period of incubation to the 

 need of normal development, and thus avoiding indefinite 

 waste of time in case of sterile or abortive eggs. 



(i) Disposition to Remain over the Eggs. The disposi- 

 tion 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 sys- 

 tem. If this suggestion seems far-fetched, when thinking 

 of birds, it will seem less so as we go back to simpler con- 

 ditions, as we find them among some of the lower inverte- 

 brate forms, which are relatively very inactive and pre- 

 disposed 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 indis- 

 position to move. That is the case with certain molluscs 

 (Crepidula), the habits and development of which have been 

 recently studied by Professor Conklin. Here full protec- 

 tion 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 Clepsine there is a manifest un- 



