worm, barely one-twentieth of an inch in length (Fig. 13, c). It has three pairs of longish bristles near the anterior end, and a single yet longer pair at the hinder extremity. These aid it in creeping over the wall of the cell. Its small head is armed with short, stiff bristles. For many days it wanders over the surface of the cell, inserting its bristly head into each minute cranny and crack. Throughout this long period it has never a bite nor sup. Probably many of them never succeed in finding a crevice by which they can effect an entrance, but those that do manage to wriggle in undergo a change, lose their bristles, and develop a minute suctorial mouth, through which the contents of the larva are absorbed into their swelling bodies (Fig. 13, D). When fully grown they are quite helpless, and unable to get out from the cell in which they are now imprisoned. For months they lie quiescent, but in the succeeding spring they pass into a pupal condition very different from that of most flies. The relatively large head is armed with strong spines; the middle region bears bristles directed backwards; the posterior end has short spines. (Fig. 13, E). Fixing itself to the interior of the cell by the latter, it strikes with its armoured head repeated blows on the walls of its prison until a breach is at last made, and sufficiently enlarged to form a suitable exit. Then the pupaskin bursts, and the imago insect emerges and flies off. At each stage of life there is the closest relation between structure and behaviour, and each is equally adapted to a biological end of which the creature has never had an opportunity of gaining any experience.

Exceedingly multifarious are the ways in which insects thus provide for the future of young they will never see. Antherophagus lives in flowers, and is believed to seize with its mandibles humble bees, which then unwittingly bear the parasitic beetle to the nests in which alone the larvæ have been found. The larvae of our common oil-beetle (Meloë) are parasitic on the bee, Anthophora. It deposits its ten thousand eggs without observable discrimination; but the active young larva instinctively seizes and attaches itself to any hairy object.

G

Thousands must go astray. They have been found on hairy beetles, flies, and bees of the wrong genus. Some, however, become thus attached to the one suitable species, and are conveyed by the Anthophora to her nest, where they promptly eat the egg she lays. It is not difficult to picture to one's self how this incompletely evolved instinct might be further perfected by natural selection, through the survival of those females which laid their eggs in the haunts of the bee-host. And such an advance in instinctive behaviour is seen in another and rarer beetle-Sitaris. Her eggs are laid in August near the entrance to a nest of the Anthophora. In September they hatch to form larvæ, which hibernate in groups till the following spring. Then they become active (Fig. 13, ▲), and attach themselves to hairy objects. Being near the Anthophora nest, there is an increased chance of their fastening upon this bee. The chance is still far from good, for if this were so, we should not find that the Sitaris laid as many as two thousand eggs. Still, on these grounds, we may presume that its chance of survival is about five times as good as that of Meloë, which lays ten thousand eggs. The larva is said generally to attach itself to a male bee, which is hatched earlier than his mate, and to pass on to the female at the nuptial period; but in any case it eventually slips on to the egg that she lays. This forms the food of the larva during the remainder of this stage of its existence. It then moults and assumes a new form, capable of feeding on the honey (Fig. 13, B); and, after further changes, becomes a pupa, and then assumes the imago condition.

In these cases the advantage is wholly on the side of the parasite. But there are cases of close relationship between insects and flowering plants where the instinctive behaviour gives rise to reciprocal benefit. The Yucca is a genus of American Liliaceous plants, with large pale sweet-smelling flowers; and these are dependent for fertilization on the instinctive behaviour of a small straw-coloured moth of the genus Pronuba. Just when the Yucca plant blossoms in the summer, the moths emerge from their chrysalis cases. They mate; and the female then flies to a flower, collects a

pellet of pollen from the anthers, proceeds to another flower, pierces the pistil with her sharp ovipositor, lays her eggs among the ovules, and finally darting to the stigma stuffs the pollen pellet into its funnel-shaped extremity (Fig. 14). If the flower be not thus fertilized

the ovules do not develop; and if the ovules do not develop the grubs which are hatched from the moth's eggs die of starvation. There are enough ovules to supply food to the grubs, and leave a balance to continue the race of Yuccas.

And

FIG. 14.-Yucca Flower and
Moth.

Whether the female moth is attracted to the flower by sight or smell, we do not know. whether the male finds the female, in the case of the Yucca moth, through scent, we are not in a position to state with certainty. It has, however, been shown that in certain moths * some odour emitted by the female is the attractive stimulus, affecting sense-organs situated on the antennæ of the male. To females confined in an opaque vessel over the mouth of which gauze was tied, the males came in numbers; but when a clear glass vessel was inverted, and sand was packed round the mouth, so as to prevent the escape of air from the interior, no males came, though the imprisoned females were clearly visible. If the antennæ of the males were either removed or coated with shellac the moths failed to notice the females even when close to them. In what way the intact male is made aware of the direction from which the scent comes, we do not know-possibly by differential stimulation in the antennæ, the moth instinctively turning in the direction of greater stimulation. It will be seen, therefore, that in the case of the behaviour of the Yucca mothbehaviour which is essential to the biological end of repro

*See A. G. Mayer "On the Mating Instinct of Moths." Ann. and Mag. of Nat. Hist., ser. 7, vol. v., Feb., 1900, p. 183.

duction-there is much detail concerning which we are ignorant. But for our present purpose the important point to notice is that the procedure of the female cannot be due to imitation; nor can it be the outcome of individually acquired experience; for the method of procedure is not gradually learnt, but is carried out without apparent hesitation the first and only time the appropriate occasion presents itself. Not only does the moth take no heed of her grubs, but they are so placed that she could not in any case ascertain by observation that only if the ovules are fertilized do her offspring thrive. She cannot possibly know what effect the stuffing of the pollen on to the stigma exercises, or indeed whether it have any effect at all. And yet generation after generation these moths collect the pollen from the anthers and bear it to the stigma. Spence's words "without knowledge of the end in view" are amply justified in this case, as in other cases of typically instinctive behaviour.

Since it is easy to hatch birds of many species in an incubator, and to rear them under conditions which not only afford facilities for observation but exclude parental influence, their study has special advantages. One can with some approach to accuracy distinguish the instinctive from the acquired factors in their behaviour.*

The callow young of such birds as pigeons, jays, and thrushes are hatched in a helpless condition, and require constant and assiduous ministration to their elementary organic needs. Most of their instincts are of the deferred type. But pheasants, plovers, moor-hens, domestic chicks, and ducklings, with many others, are active soon after birth, and exhibit powers of complex co-ordination, with little or no practice of the necessary limb-movements. They walk

* Some of the observations on which the summary of results given in this section are founded are presented in some detail in "Habit and Instinct," pp. 29-100.

and balance the body so soon and so well as to show us that this mode of procedure is congenital, and has not to be gradually acquired through the guidance of experience. Young water-birds swim with neat and orderly strokes the very first time they are gently placed in water. Even little chicks a day or two old can swim well. Dr. Thorndike, who draws attention to this fact,* appears to accept the view,

suggested by Dr. Bashford Dean, that the movements are not those of swimming but only of running. I have carefully watched the action through the glass walls of a tank and compared it with that of a young moor-hen. In the two cases it is quite similar in type, and the type appears to be different from that of running, though it is perhaps hard to distinguish

*Psychological Review, May, 1899, p. 286.