Imitation is probably a by no means unimportant factor in the development of habits and instincts. Mr. A. R. Wallace, in his "Contributions to the Theory of Natural Selection," contends that the nest-building habit in birds is, to a large extent, kept constant by imitation. The instinctive motive is there, but the stereotyped form is maintained through imitation of the structure of the nest in which the builders were themselves reared. Mr. Weir, however, writing to Mr. Darwin, in 1868, says in a letter, which Mr. Romanes quotes,[JR] "The more I reflect on Mr. Wallace's theory, that birds learn to make their nests because they have themselves been reared in one, the less inclined do I feel to agree with him.... It is usual with canary-fanciers to take out the nest constructed by the parent birds, and to place a felt nest in its place, and, when the young are hatched and old enough to be handled, to place a second clean nest, also of felt, in the box, removing the other. This is done to prevent acari. But I never knew that canaries so reared failed to make a nest when the breeding-time arrived. I have, on the other hand, marvelled to see how like a wild bird's the nests are constructed. It is customary to supply them with a small set of materials, such as moss and hair. They use the moss for the foundation, and line with the finer materials, just as a wild goldfinch would do, although, making it in a box, the hair alone would be sufficient for the purpose. I feel convinced nest-building is a true instinct." On the other hand, Mr. Charles Dixon, quoted[JS] in Mr. Wallace's "Darwinism," speaking of chaffinches which were taken to New Zealand and turned out there, says, "The cup of the nest is small, loosely put together, apparently lined with feathers, and the walls of the structure are prolonged for about eighteen inches, and hang loosely down the side of the supporting branch. The whole structure bears some resemblance to the nests of the hang-birds (Icteridæ), with the exception that the cavity is at the top. Clearly these New Zealand chaffinches were at a loss for a design when fabricating their nest. They had no standard to work by, no nests of their own kind to copy, no older birds to give them any instruction, and the result is the abnormal structure I have just described."
There is more evidence in favour of the view that the song of birds is, in part at least, imitative. That it has an innate basis is certain; and that it may be truly instinctive is shown by Mr. Couch's observation of a goldfinch which had never heard the song of its own species, but which sang the goldfinch-song, though tentatively and imperfectly. On the other hand, imitation is undoubtedly a factor. The Hon. Daines Barrington says (1773), "I have educated nestling linnets under the three best singing larks—the skylark, woodlark, and titlark—every one of which, instead of the linnet's song, adhered entirely to that of their respective instructors. When the note of the titlark linnet was thoroughly fixed, I hung the bird in a room with two common linnets for a quarter of a year. They were in full song, but the titlark linnet adhered steadfastly to that of the titlark." Mr. Wallace, who quotes this, adds,[JT] "For young birds to acquire a new song correctly, they must be taken out of hearing of their parents very soon, for in the first three or four days they have already acquired some knowledge of the parent's notes, which they afterwards imitate." Dureau de la Malle, as quoted by Mr. Romanes,[JU] describes how he taught a starling the "Marseillaise," and from this bird all the other starlings in a canton to which he took it are stated to have learned the air!
That dogs, monkeys, and other mammalia have powers of imitation needs no illustration. And when we remember that it is only the imitation of strange and unusual actions that arrests our attention, while the imitation of normal activities is likely to pass unnoticed, we may, I think, fairly surmise that imitation is by no means an unimportant factor in the acquisition and development of habits. And where the young animal is surrounded during the early plastic and imitative period of life by its own kith and kin, imitation will undoubtedly have a conservative tendency.
The education of young animals by their parents has also a conservative tendency. Mr. Spalding's observations show that the flight of birds is instinctive; but the parent birds normally aid the development of the instincts by instruction. Ants, as we have seen, are instructed in the business of ant-life. Dogs and cats train their young. And Darwin tells us, on the authority of Youatt,[JV] that lambs turned out without their mothers are very liable to eat poisonous herbs.
We may say, then, with regard to the influence of intelligence on instinctive activities, that it may lead them to vary along certain definite lines of increased adaptation; that it may, in some cases, lead them to vary along divergent lines, and hence tend to render stereotyped instincts more plastic; and that, through imitation and instruction, it may tend to render instinctive habits more uniform in a community, and hence, if the habits are tending to vary under changed circumstances in a given direction, may tend to draw the habits of all the members of the community in that given direction.
And with regard to the more general question of the variation of habits and instincts, we may say that, in addition to those variations in the origin and direction of which intelligence is a factor, there are other variations which take their origin without the influence of intelligence under the stress of changing circumstances, and yet others which may arise as we say "fortuitively" or "by chance," that is, from some cause or causes whereof we are at present ignorant, and which do not appear to be evoked directly by the stress of environing circumstances.
Granting, however, the existence of these variations in whatsoever way arising, and granting the influence of natural selection, of sexual selection, and perhaps of the inheritance of individually acquired modifications, those variations which are for the good of the race or species in which they occur will have a tendency to be perpetuated, while those which are detrimental will be weeded out and will tend to disappear.
Passing on now to consider the characteristics of those activities which we term "intelligent," we may first notice what Mr. Charles Mercier, in "The Nervous System and the Mind," calls the four criteria of intelligence. Intelligence is manifested, he says, first, in the novelty of the adjustments to external circumstances; secondly, in the complexity; thirdly, in the precision; and fourthly, in dealing with the circumstances in such a way as to extract from them the maximum of benefit.
Now, I think it is clear that, when it is our object to distinguish intelligent from instinctive activities, the precision of the adjustment cannot be regarded as a criterion of intelligence. Many instinctive acts are wonderfully precise. The sphex is said to stab the spider it desires to paralyze with unerring aim in the central nerve-ganglion. Other species, which paralyze crickets and caterpillars, pierce them in three and nine places respectively, according to the number of the ganglia. And yet this seems to be a purely instinctive action. So, too, to take but one more example, there is surely no lack of precision in the cell-making instinct of bees. We may say, then, that, granting that an action is intelligent, the precision of the adjustment is a criterion of the level of intelligence; but that, since there may be instinctive actions of wonderful precision, this criterion is not distinctive of intelligence. Nay, more, there are many reflex actions of marvellous precision and accuracy of adjustment; and there can be no question of intelligence, individual or ancestral, in many of these.
Nor can we regard prevision (which is sometimes advanced as a criterion of intelligence) as specially distinctive of intelligent acts regarded objectively in the study of the activities of animals. For, as we have already seen, there are many instincts which display an astonishing amount of what I ventured to term "blind prevision"—instance the instinctive regard for the welfare of unborn offspring, and the instinctive preparation for an unknown future state in the case of insect larvæ.