The same distinguished zoologist also refers to this Family remains designated by him Agnopterus, and those of the "Elornis" (properly Helornis) of M. Aymard (cf. FoSSIL BIRDS).

FLAX-BIRD, the North-American Goldfinch or "Yellow Bird," Chrysomitris tristis (congeneric with our SISKIN), so "called in the back parts of Carolina" as Latham (Gen. Hist. B. vi. p. 120) was informed by Abbot; but the name seems to have dropped out of use.

FLICKER, one of the most characteristic, common, and conspicuous birds of the greater part of North America, the Goldenwinged WOODPECKER of books, the Picus auratus of Linnæus, and

COLAPTES. (After Swainson.)

Colaptes auratus of modern ornithology. Its habits have been well described by Wilson, Audubon, and other writers,

but there is no space here to dwell upon them, engaging as the topic is, for the mention of this bird suggests a more important theme. Widely distributed as it is from the Atlantic coast, so far southward as Louisiana, to Canada, and thence across the Rocky Mountains, and still further northward to Alaska, its place is taken on the greater part of the Pacific side by a species which, avoiding Southern California, reaches the tablelands of Mexico-a species more brilliantly tinted, for ruby appears in its plumage instead of gold, the C. mexicanus or rubricatus 1 of authors. But in an intervening broad belt running north-westward from Texas to British Columbia there occur birds presenting almost every combination of the distinctive coloration of the two species just named,2 and though one of these intermediate specimens had been long before figured and described as C. ayresi by Audubon (B. Amer. vii. p. 348, pl. 494), yet Baird was so much persuaded that all these puzzling birds were hybrids, that he used (Expl. &c. Railroad Route, ix. p. 122) the name C. hybridus to cover the whole of them.3 It must be admitted that

1 By some writers identified with the P. cafer of Gmelin, founded on Latham's description of a specimen said to have come from South America; but most likely the locality assigned is wrong.

2 The series contained in the Museum of the Smithsonian Institution in 1857 was in that year shewn to me and descanted upon by my highly esteemed friend the late Prof. S. F. Baird. He did not convince me of the truth of his views, and I afterwards saw greater reason to doubt their correctness; but they were probably the only views in those days consonant with philosophy to any one not in the confidence of Mr. Darwin, whose secret was not revealed till the next year.

3 Cassin at that time was inclined to believe that they could be broken up into several distinct "species"; but I do not know that he ever published this opinion.

this is a view which cannot at present be disproved; but it also must be admitted that neither has it been proved. Mr. J. A. Allen was supposed to explain the difficulty (Bull. Mus. Harvard, iii. p. 118) as arising from climatic influences, while Dr. Coues was formerly (B. North-West, p. 294) inclined to support that view, but subsequently (Key N.-Am. B. p. 492) followed Mr. Ridgway's suggestion (Orn. Fortieth Parallel, p. 556, note) that these birds might be "remnants of a generalized form." If so, a case would be found analogous to that presented by certain forms of Coracias (ROLLER), and a good many of the Phasianidæ, to say nothing of other groupsthough strictly intermediate forms are not often met with. So much has been written on what is called the "interbreeding" of species by persons who seem ignorant of the fact that all specialized forms must have sprung from more generalized ancestors, that the careful zoologist will abstain from invoking a theory of "interbreeding" to account for every difficulty that presents itself in the differentiation of species. Granted that most of these generalized forms are by this time become extinct, there is no reason, even allowing for the going on of specialization, why some of them should not still exist, and thus such forms of Colaptes, Coracias, Euplocamus, and Phasianus survive to this time. Against this view, however, may be set the fact that examples of C. ayresi or hybridus offer some characters so very pronounced that those who favour the hybrid origin have apparently a strongish case. But then it may be reasonably alleged that zoologists, to their shame, are so absolutely ignorant of the laws that govern hybridity in animals, that no argument can be founded on a presumption that has positively no foundation-for no zoologist has as yet carried out any such series of experiments as has again and again been done by botanists to the very great advancement of their study. Consequently the phænomena of hybridity in animals can only be interpreted-and possibly wrongly interpreted-by those observed in plants. Among the few experiments hitherto made in regard to Birds, some unmistakably shew how strongly the principle of Reversion works.

Another interesting fact relating to the genus is that at least one of the South-American members of it, C. agricola, inhabiting the treeless plains of La Plata and Patagonia, has succeeded in accommodating itself to circumstances,—as recorded among others by Darwin (Origin of Species, chap. vi.) under the name of C. campestris, which seems rightfully to belong to a more northern form. Since Azara's time it has been known to frequent the open country, seek

1 For instance, an example may be all mexicanus on one side and all auratus on the other!

2 Since this was in type, Mr. Allen has published (Bull. Am. Mus. N. H. iv. pp. 21-44) the results of an elaborate investigation which he says "tend strongly to confirm Baird's startling hypotheses of hybridization on a grand scale."

ing its food on the ground, and to breed in holes which it excavates in cliffs, the banks of streams, or in old walls; but latterly, according to Mr. Hudson (Argent. Ornithol. ii. p. 25), it has taken to make its nest in trees, reverting presumably to the habit of its ancestors. The fact rests on the best of evidence, but any inference is open to criticism. Curiously enough, a very similar state of things is presented by an apparently cognate bird from South Africa, Geocolaptes olivaceus or arator, which bears a strong superficial resemblance and possibly (though this has not yet been ascertained) a deepseated affinity to the American forms. This Woodpecker, as Mr. Layard remarks (B. S. Afr. p. 239), "never pecks wood, but bores its way into the banks of rivers, sides of hills, or the walls of mudbuildings, in search of its prey, and for a home for its young." Mr. Buckley states (Ibis, 1874, p. 369) that in Natal he never noticed it among trees; but found it on the open hills and sitting among stones. Considering how few Woodpeckers there are in the Ethiopian Region, the occurrence at its southern extremity of this simulacrum of a New-World type raises more than one question of the deepest interest.

FLIGHT.1 Birds have three chief modes of flight, each differing from the others in certain important particulars. These areI. By gliding or skimming, supported on the extended wings, which do not flap up and down. Most probably all birds that fly can move in this manner. It requires a certain velocity of motion. of the bird through the air (relative velocity), which is acquired (1) by previous strokes of the wings, (2) by descending from a higher to a lower level, or (3) by commencing flight in a wind of sufficient velocity.

II. By active strokes of the wings. The manner in which this mode of progression is carried out varies in detail in different birds, and in the same bird at different times, but its main features appear to be the same for all.

III. By sailing or soaring with motionless extended wings. This appears to be only possible for certain birds, and is not described as taking place except in a wind of a certain minimum velocity, and differs from ordinary gliding in the fact that the bird does not necessarily lose either in velocity or in vertical position, as a result of the resistance of the air to the bird's passage through it.

1 I am indebted for this article to my colleague Prof. Roy, who remarks that, in it, he has "sought to avoid inaccuracy of fact or method of statement, the main object being to put the matter in as simple a form as possible, so as not to confuse the non-scientific reader. The references given to the most important authorities on the subject will enable those who wish to pursue it further to do So "-A. N.

Before analyzing these modes of aerial locomotion, it is desirable to refer to some of the conditions under which birds are placed, since these must be taken into account if it be desired to understand the problems of flight. What, in the first place, is known about the relation between the weight of birds and the area of their wings, and how do birds differ from one another in this respect? This subject has been carefully studied by Müllenhoff1 and others. It has been found that the relation of the wing- or rather sail-area to the weight of the bird varies greatly. As might be expected, the greater the sail-area, the more powerful, other things being equal, is the flight.

Another matter in which birds differ greatly is the strength of the muscles which move the wings. It may be assumed that the strength of these muscles corresponds with their weight. The relation between the weight of the pectoral muscles and that of the whole bird has been investigated among others by Legal and Reichel, who found that the pectoral muscles weigh on an average about one-sixth of the whole bird; but that in different types of birds there may be considerable difference in this respect. For instance, in a House-Pigeon the proportion was 45 per cent, while in a Herring-Gull it was only 16 per cent. Some birds therefore have much more powerful wing-muscles than others.

The shape of the wing, moreover, varies considerably also in different birds. Some, like the Swallow, have long and narrow wings, while others, like the Quail, have short and broad wings. The wings of some soaring birds, as Eagles and Vultures, are rounded at the points, and the primary feathers are separated from one another at their tips, giving a notched appearance to the end of the extended wings. A typically flying bird, such as a Falcon, on the other hand, has pointed wings with little separation of the tips of the primary feathers. These differences correspond to differences in the power and mode of flight.

Birds, as well as all other animals that fly, may be divided into categories according (a) to the ratio between the sail-area and the weight, (b) to the strength of the pectoral muscles, and (c) to the shape of the wings. Of these categories or "types" Müllenhoff

1 "Die Grösse der Flugflächen." Archiv für die gesammte Physiologic (Pflüger's), xxxv. (1885) pp. 407 et seqq. Müllenhoff follows Harting, Legal and Reichel, Marey, and others in estimating the ratio between the sail-area of Birds and their weight by the formula A/P =σ, in which A is the area (in square centimetres) of the out-stretched wings and tail as well as of the body, spread out on a flat surface, while P is the weight of the bird (in grammes). The values found for σ in different birds range from 2.85 (Golden-Eye) to 6·735 (Barn-Owl).

2 Verhandlungen der Schlesischen Gesellschaft für vaterländ-Cultur. Breslau:

distinguishes six,-namely the Quail, Pheasant, Sparrow, Swallow, Vulture, and Gull.

In considering birds from the point of view of their flying capacities, it should be kept in mind that their anatomical structure is such as to give them great stability in the air. The wings are attached to the highest part of the thorax, so that the centre of gravity of the bird is as low as possible beneath the centre of suspension on the wings, and also that the same object is assisted by the light organs, viz. the lungs and air-sacs, being placed high, while the heavy organs of digestion, and above all the heavy pectoral muscles, are placed as low as possible. The shape of a bird's body, moreover, is such that it offers little resistance to its passage through the air.

We must here say something about the resistance offered by the air to the passage of a body through it. As Newton first shewed, this resistance increases with the square of the velocity of the body. It increases also directly with the sectional area at right angles with the axis of motion (the geometrical form of the body being similar), and this velocity and area multiplied by one another and by a numerical coefficient gives the resistance.1

Of the greatest importance for the flight of birds is the fact that the resistance offered to the motion of a flat body in a direction at right angles to the plane of its surface is very greatly increased if it be made to move at the same time through the air rapidly in a direction parallel with its surface.

As a clear comprehension of this matter is important, an illustration may be given from Sir George Cayley.2 Supposing a flat surface, such as a piece of cardboard, with a superficial area of one foot and inclined at an angle of six degrees to the line of movement, be carried forwards horizontally, as in the case of the wings of a gliding bird, it is found that with a rapidity of 23.6 feet per second the pressure perpendicular to its surface is 4 of a pound, while if the speed be increased to 27.3 feet per second, it is equal to one pound. A Rook, whose weight and wing-area were found by Cayley to be, roughly speaking, in the ratio of one pound to the square foot, would therefore be able to glide horizontally whenever it had a velocity of 27.3 feet. But a Rook usually flies

1 Expressed more exactly, the resistance, R, offered by the air to the passage of рп sin 2 a a flat surface through it may be stated as R = K SV 2 ; K being the

4 + π sin a

numerical coefficient, S the surface, V the velocity, p the density of the air, and a the angle of the surface with the line of motion. It must be understood here that the resistance is to the forward movement of the surface, i.e. the resistance in the line of motion.

2 "On Aërial Navigation." Journal of Natural Philosophy, Chemistry, and the Arts (Nicholson's), xxiv. p. 164 (1809).