has the merit of corresponding with the minimum expenditure of force. To admit of the exit of the whole series, if the string consists of n cells, there are originally n partitions to be perforated. There might even be one more, owing to a complication which I disregard. There are, I say, at least n partitions to be perforated. Whether each Osmia pierces her own, or whether the same Osmia pierces several, thus relieving her neighbours, does not matter to us: the sum-total of the force expended by the string of Bees will be in proportion to the number of those partitions, in whatever manner the exit be effected.

But there is another task which we must take seriously into consideration, because it is often more troublesome than the boring of the partition: I mean the work of clearing a road through the wreckage. Let us suppose the partitions pierced and the several chambers blocked by the resulting rubbish and by that rubbish only, since the horizontal position precludes any mixing of the contents of different chambers. To open a passage for itself through these rubbish-heaps, each insect will have the smallest effort to make if it passes through the smallest possible number of cells, in short, if it makes for the opening nearest to it. These smallest individual efforts amount, in the aggregate, to the smallest total effort. Therefore, by proceeding as they did in my experiment, the Osmiae effect their exit with the least expenditure of energy. It is curious to see an insect apply the 'principle of least action,' so often postulated in mechanics.

An arrangement which satisfies this principle, which conforms to the law of symmetry and which possesses but one chance in 512, is certainly no fortuitous result. It is determined by a cause; and, as this cause acts invariably, the same arrangement must be reproduced if I renew the experiment. I renewed it, therefore, in the years that followed, with as many appliances as I could find bramble-stumps; and, at each new test, I saw once more what I had seen with such interest on the first occasion. If the number be even—and my column at that time consisted usually of ten—one half goes out on the right, the other on the left. If the number be odd—eleven, for instance—the Osmia in the middle goes out indiscriminately by the right or left exit. As the number of cells to be traversed is the same on both sides, her expenditure of energy does not vary with the direction of the exit; and the principle of least action is still observed.

It was important to discover if the Three-pronged Osmia shared her capacity, in the first place, with the other bramble-dwellers and, in the second, with Bees differently housed, but also destined laboriously to cut a new road for themselves when the hour comes to quit the nest. Well, apart from a few irregularities, due either to cocoons whose larva perished in my tubes before developing, or to those inexperienced workers, the males, the result was the same in the case of Anthidium scapulare. The insects divided themselves into two equal batches, one going to the right, the other to the left. Tripoxylon figulus left me undecided. This feeble insect is not capable of perforating my partitions; it nibbles at them a little; and I had to judge the direction from the marks of its mandibles. These marks, which are not always very plain, do not yet allow me to pronounce an opinion. Solenius vagus, who is a skilful borer, behaved differently from the Osmia. In a column of ten, the whole exodus was made in one direction.

On the other hand, I tested the Mason-bee of the Sheds, who, when emerging under natural conditions, has only to pierce her cement ceiling and is not confronted with a series of cells. Though a stranger to the environment which I created for her, she gave me a most positive answer. Of a column of ten laid in a horizontal tube open at both ends, five made their way to the right and five to the left. Dioxys cincta, a parasite in the buildings of both species of Mason-bees, the Chalicodoma of the Sheds and the Chalicodoma of the Walls (Cf. "The Mason-bees" by J. Henri Fabre, translated by Alexander Teixeira de Mattos: passim.—Translator's Note.), provided me with no precise result. The Leaf-cutting Bee (Megachile apicalis, SPIN. (Cf. Chapter 8 of the present volume.—Translator's Note.)), who builds her leafy cups in the old cells of the Chalicodoma of the Walls, acts like the Solenius and directs her whole column towards the same outlet.

Incomplete as it is, this symmetry shows us how unwise it were to generalize from the conclusions to which the Three-pronged Osmia leads us. Whereas some Bees, such as the Anthidium and the Chalicodoma, share the Osmia's talent for using the twofold exit, others, such as the Solenius and the Leaf-cutter, behave like a flock of sheep and follow the first that goes out. The entomological world is not all of a piece; its gifts are very various: what one is capable of doing another cannot do; and penetrating indeed would be the eyes that saw the causes of these differences. Be this as it may, increased research will certainly show us a larger number of species qualified to use the double outlet. For the moment, we know three; and that is enough for our purpose.

I will add that, when the horizontal tube has one of its ends closed, the whole string of Osmiae makes for the open end, turning round to do so, if need be.

Now that the facts are set forth, let us, if possible, trace the cause. In a horizontal tube, gravity no longer acts to determine the direction which the insect will take. Is it to attack the partition on the right or that on the left? How shall it decide? The more I look into the matter, the more do my suspicions fall upon the atmospheric influence which is felt through the two open ends. Of what does this influence consist? Is it an effect of pressure, of hygrometry, of electrical conditions, of properties that escape our coarser physical attunement? He were a bold man who should undertake to decide. Are not we ourselves, when the weather is about to alter, subject to subtle impressions, to sensations which we are unable to explain? And yet this vague sensitiveness to atmospheric changes would not be of much help to us in circumstances similar to those of my anchorites. Imagine ourselves in the darkness and the silence of a prison-cell, preceded and followed by other similar cells. We possess implements wherewith to pierce the walls; but where are we to strike to reach the final outlet and to reach it with the least delay? Atmospheric influence would certainly never guide us.

And yet it guides the insect. Feeble though it be, through the multiplicity of partitions, it is exercised on one side more than on the other, because the obstacles are fewer; and the insect, sensible to the difference between those two uncertainties, unhesitatingly attacks the partition which is nearer to the open air. Thus is decided the division of the column into two converse sections, which accomplish the total liberation with the least aggregate of work. In short, the Osmia and her rivals 'feel' the free space. This is yet one more sensory faculty which evolution might well have left us, for our greater advantage. As it has not done so, are we then really, as many contend, the highest expression of the progress accomplished, throughout the ages, by the first atom of glair expanded into a cell?