Laws and Hypotheses for Behavior aay 
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: 
(x) 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 sare 
(1) 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 fnimals remaining over their eggs, and thus 
shielding them from harm, from sheer inability or indis- 
position to mo That is the case with certain molluscs 
(Crepidula), thehabits 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- 
