2. Means Rest and Incidental Protection 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 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: (1) the disposition to remain with or over the eggs; (2) the disposition to resist and drive away enemies; and (3) periodicity. The birds brought all these elements along in their congenital equipment, 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.
(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 Professor Conklin. 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 Clepsine there is a manifest unwillingness to leave the eggs, showing 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.”[45]
Professor Whitman proceeds to study the ‘Disposition to Resist Enemies’ and the ‘Periodicity’ in the same genetic way.
The most important of all original abilities is the ability to learn. It, like other capacities, has evolved. The animal series shows a development from animals whose connection-system suffers little or no permanent modification by experience to animals whose connections are in large measure created by use and disuse, satisfaction and discomfort.
Some of this development can be explained without recourse to differences in mere power to learn, by the fact that the latter animals are given greater stimuli to or rewards for learning. But part of it is due to differences in sheer ability to learn, that is, in the power of equally satisfying conditions to strengthen or of equally annoying conditions to weaken bonds in the animals’ connection-systems. This may be seen from the following simple and partial case:—
Call 1 and 2 two animals.
Call C₁ and C₂ the internal conditions of the two animals except for their connection-systems, each being the average condition of the animal in question.
Call S₁ and S₂ two external states of affairs, each being near the indifference point for the animal in question,—that is, being one which the animal does little to either avoid or secure.
Call G₁ and G₂ two responses which result in O₁ and O₂ the optima or most satisfying state of affairs for 1 and 2.