fishes are astonishingly great. A blow from the head of a sperm-whale may endanger a strong ship, and the sword of a sword-fish has been driven through the oak-planks of a vessel more than twelve inches in thickness.

When the flying-fish rises in the air, it is by the momentum it attains in the water by the lashing of its fins and tail. Fairly in the air, its wings give it support, and, in the opinion of Dr. Pettigrew, act as true pinions within certain limits, but are too small to sustain the creature indefinitely.

The transition from swimming to progression in the air is natural and easy. The method by which the flying-fish rises from the water is similar to that of the albatross, that prince of flying-birds, and, indeed, to perhaps all other birds, when in the act of taking flight upon the water. Momentum is obtained by rushing forward with both feet and wings. The albatross frequently goes in this way many rods before it is fairly launched upon the air. Then, with powerful strokes, it rises above the waves. Its expenditure of force is chiefly in rising, when, without further effort, except to screw and unscrew its pinions upon the wind, it floats facing the gale. For more than an hour it will sail with wings apparently motionless, and it seems most at rest when the winds are highest. In this case it is sustained by the momentum it attained, and the wonderful kite-like position and adjustment of its wings. But, it manifestly could not maintain its position in this way, if moving before the wind, or in a perfectly calm atmosphere. The wings must then be called into play to afford lifting as well as propelling power. The momentum must be supplied.

Birds rise from the ground most readily facing the wind, but usually run or leap, and the wings, by vigorous strokes, continue the impulse secured. With the first down-stroke of the wings the body is lifted, and some velocity attained; when the wings rise, the body falls somewhat, but is at the same time advancing. This rise and fall of the body in flying continue, and the body, in progressing, undulates above and below a given line. In the flight of birds with large wings and slow stroke, it is easily observed. The illustration, Fig. 11, shows

the positions of the wings of the gull in the course of a complete oscillation:

FIG. 11.

SHOWING THE MORE OR LESS PERPENDICULAR DIRECTION OF THE STROKE OF THE WING IN THE FLIGHT OF THE GULL.

When the down-stroke is completed the bird has been raised, but is lowered again when the wings have attained their maximum elevation. Thus it is seen how directly gravity aids in flight. The body is the weight; the wings are long levers attached to it at one end; the air is the fulcrum. Fig. 12 shows the undulatory track of a flyingbird:

FIG. 12.

6a

BIRD at a; down-stroke of the wing, b, lifts the bird to c-the track of the bird being in direction of the arrows.

The instant the descent of the wing begins, the body moves upward and forward; but it is shown by the author that some forward motion results also from the up-stroke. Certain it is that the upward movement must not counteract the other. There is no provision for

LEFT WING OF THE ALBATROSs.-d, e, f, Anterior or thick margin of pinion; b, a, c, Posterior or thin margin, composed of the primary (b), secondary (a), and tertiary (c) feathers.

waste of energy. The form of the upper surface of the wing is convex, the under surface being concave. The value of this will be apparent, as the Duke of Argyle suggests, if we attempt to move the

concave or convex side of an umbrella against the wind; one side holds the air, the other discharges it. The wing of the albatross shows how completely the feathers are adjusted, on the upper side, to avoid any hold upon the ai .

This arrangement, with the flexibility and screw-like motion of the wings of the gull, shown in Fig. 14, explains the exceedingly small resistance experienced in the upward movement, and also the forward impetus which it communicates.

FIG. 14.

SHOWS THE TWISTED LEVERS OR SCREWS FORMED BY THE WINGS OF THE GULL.

It is in the down-stroke, or, as Dr. Pettigrew insists, in the beginning of the down-stroke, that force is chiefly expended. This movement is essentially a muscular act, and by this force alone no bird could sustain long-continued flight. The lark, whose flight is upward, soon descends to the earth. It lifts itself against gravity, simply by expenditure of vital force. But, the moment forward motion is attained, other forces relieve the strain upon the pinions, and their inclined surfaces convert gravity into a propelling power. It is obvious, however, that flight is attended with considerable muscular exertion. Migrating birds alight in unsuitable positions for rest, but the swallow will fly 1,000 miles in a single journey, and the condor attains an altitude of six miles.

The heron will strike the air 60 times in a minute, which, with 60 up-strokes, gives 120 movements, and this is continued through long

FIG. 15.

THE GRAY HERON IN FULL FLIGHT.

flights; and the same is true of many ducks and land-birds which strike the air with extreme and apparently exhausting rapidity. So swift are the motions of the wings of the humming-bird that they produce only a blurred spot before the eye.

That wings act as true kites, when in motion, is a familiar obser

vation, but they are kites which continually change their surfaces and position in respect to the air, which artificial kites do not. An important difference between them is the rigidity of the one and the wonderful flexibility of the other. The kite rises as its oblique surface is pressed against the elastic air; the same is true of the wing. But the wing rotates, so that the proper obliquity of its parts is continually maintained; it rolls on and off the wind; it rotates not only throughout its length, but in each of its parts. The quills, which are convex, rotate, and present closed or oblique surfaces, which hold or discharge the air.

We have space for but one more of the numerous diagrams and figures which Dr. Pettigrew has prepared, illustrating the phenomena of flight. Fig. 16 is of the extended wing of a partridge, seen from beneath and from behind.

FIG. 16.

CURVES IN THE WING OF A PARTRIDGE IN FLIGHT.

The wings, when flexed and extended in flight, assume curved surfaces, which change at each instant and carve the air, as does the tail of the fish the water, into complex wave-lines; and such is the structure of the wing that these results are inevitable when it is put in motion. "The muscles, bones, ligaments, and feathers, are so adjusted with reference to each other that, if the wing is moved at all, it must move in the proper direction." The bird no more thinks of its motions in flying than we do of ours in walking; the actions are mechanical and instinctive. An opinion long prevailed that heated air in the hollow bones of the bird gave it buoyancy and power of flight. This is shown to be a fallacy.

Three principal forces are expressed in flight: muscular and elastic force of the wing, weight of the body, and recoil of the air. By the mechanical structure of the wing, these forces act, react, and combine. Thus birds traverse the aërial ocean; the wild-goose drives his train along invisible tracks; the albatross and petrel are at home in the gale, undisturbed by its clamor; and the condor, with easy motion, treads with his pinions the elastic floors of the upper air.

ment.

The more rapid the strokes of the wing the greater the achieveNot so with one of the most ingenious of human contrivances for progression. The screw, if urged beyond a certain velocity, holds and carries with it the water, and its propelling power is lost. It wants the flexibility of the wing and the fin-the adaptation is not complete.

REPLIES TO THE QUARTERLY REVIEWERS.

WITH

BY HERBERT SPENCER.

WITH the concluding paragraph of the previous article replying to criticisms I had hoped to end, for a long time, all controversial writing. But, while it was in the printer's hands, two criticisms, more elaborate than those dealt with above, made their appearance; and, now that the postponed publication of this latter half of the article affords the opportunity, I cannot, without risking misinterpretations, leave these criticisms unnoticed.

Especially do I feel called upon by courtesy to make some response to one who, in the Quarterly Review, for October, has dealt with me in a spirit which, though largely antagonistic, is not wholly unsympathetic, and who manifestly aims to estimate justly the views he opposes. In the space at my disposal, I cannot of course follow him through all the objections he has urged. I must content myself with brief comments on the two propositions he undertakes to establish. His enunciation of these runs as follows:

"We would especially direct attention to two points, to both of which we are confident objections may be made; and, although Mr. Spencer has himself doubtless considered such objections (and they may well have struck many of his readers also), we nevertheless do not observe that he has anywhere noticed or provided for them.

"The two points we select are:

"1. That his system involves the denial of all truth.

"2. That it is radically and necessarily opposed to all sound principles of morals."

On this passage, ending in these two startling assertions, let me first remark that I am wholly without this consciousness the reviewer ascribes to me. Remembering that I have expended some little labor in developing what I conceive to be a system of truths, I am somewhat surprised by the supposition that "the denial of all truth" is an implication which I am "doubtless" aware may be alleged against this system. Remembering, too, that by its programme this system is shown to close with two volumes on "The Principles of Morality," the statement that it is "necessarily opposed to all sound principles of morals" naturally astonishes me, and still more the statement that I am doubtless conscious it may be so regarded. Saying thus much by way of repudiating that latent skepticism attributed to me by the reviewer, I proceed to consider what he says in proof of these propositions.

On those seeming incongruities of Transfigured Realism commented on by him, I need say no more than I have already said in reply to Mr. Sidgwick, by whom also they have been alleged.

I