324 BIOLOGICAL LECTURES. 



infinite amount of chancing must have. followed before the first 

 "cuddling" became a habit, and the habit a perfect instinct! 

 We are driven to such preposterous extremities as the result 

 of taking a purely casual feature to start with. Incubation 

 supplies the needed heat, but that is an incidental utility that 

 has nothing to do with the nature and origin of the instinct. 

 It enables us to see how natural selection has added some 

 minor adjustments, but explains nothing more. For the 

 real meaning of the instinct we must look to its phyletic 

 roots. 



If we go back to animals standing near the remote ancestors 

 of birds, to the amphibia and fishes, we find the same instinct 

 stripped of its later disguises. Here one or both parents sim- 

 ply remain over or near the eggs and keep a watchful guard 

 against enemies. Sometimes the movements of the parent 

 serve to keep the eggs supplied with fresh water, but aeration 

 is not the purpose for which the instinct exists. 



2. Means Rest and Incidental Pi'otection to Offspring. — 

 The instinct is a part of the reproductive cycle of activities, 

 and always holds the same relation in all forms that exhibit it, 

 whether high or low. It follows the production of eggs or 

 young, and means primarily, as I believe, rest with 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 interest to 

 one as to the other. 



3. Essential Elements of the Instinct. — Every essential ele- 

 ment in the instinct of incubation was present long before the 

 birds and eggs arrived. These elements are : (i) the disposi- 

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

 resist and to drive away enemies ; and (3) periodicity. The 

 birds brought all these elements along in their congenital equip- 

 ment, and added a few minor adaptations, 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. 



