Two sections of the canal pass through dry land; the one from the North Sea to Wijkermeer, at Valsen, 6540 yards long; and the other across the peninsula of Beutzenhuisen, 875 yards in length. The portion of the canal passing through the Wijkermeer has a length of 4779 yards, and that through the Western Ij one of 13,725 yards.

The canal is formed with a bottom width of 88 feet 6 ins., and a surface width of 207 feet, and when finally completed it will be over 23 feet in depth. These dimensions continue throughout 22,966 yards in length of the canal, measured from the North Sea, after which the width gradually increases until it joins the "diep " near Amsterdam : here the dykes turn away from the canal, the northern one running in a straight line to Buikslooterham, and the southerly one in a curved line to Amsterdam.

The earth excavated from the land sections of the canal is conveyed in barges, and deposited on the muddy bottom of the lake to form dykes on either side, where it passes through the Wijkermeer and the Ij. These deposits are then covered with clay, and as soon as the bank appears above the surface of the water, and is formed, the slopes are protected from the wash of the waves with fascines. The canal channel is then dredged between the banks so formed, for which purpose steam dredgers are employed with selfacting discharging apparatus. This apparatus consists of a vertical cylinder fitted on to the steam dredger, into which the excavated mud is thrown. On the lower end of this cylinder a horizontal centrifugal pump-wheel, of about 40 ins. in diameter, works, which is driven by the engine of the steam dredger, at a speed of 230 revolutions a minute, forcing the mud from the cylinder, in a semi-fluid state, into a floating tube, through which it is carried away. The floating tubes consist of wooden cylinders of about 50 feet in length and 15 inches in diameter, joined together by flexible leather couplings. These float on the surface of the water for distances of from 750 to 800 feet, the ends being carried through and sometimes over the previously-made sandy dykes, behind which a jet of muddy water is discharged, containing from 40 to 50 per cent of solid matter, which thus becomes deposited over a considerable surface. From 1200 to 1500 tons of sand is raised by one dredger, with a 20-horse power engine, per day.

In order to connect the different navigation and drainage locks and sluices along the borders of the Ij with the main canal, several branch canals have been constructed, of varying sections, and having an aggregate length about

equal to that of the main canal. The total length of dykes constructed is about 40 miles, and the excavation from the canals has amounted to about 13 millions of cubic yards of sand and mud.

The spaces outside the canal, being pumped dry, become suitable for meadows, or, as they are called" polders." In order to keep them dry from percolation or drainage, pumping-engines are placed at certain distances along the canal, which drain them and empty the water into the canal. The total amount reclaimed, and to be reclaimed, is 12,450 acres. These polders continue along the canal side until the sand dunes are reached at 3 miles from the west coast.

The canal is crossed at two points by railway bridges, but the most important engineering works are the locks at either end of the canal, which deserve a brief description here.

As has been already stated, the North Sea locks are placed 1300 yards from the western end of the canal. In these, special precautions were necessary to protect them from high tides. The locks are unusually high and strong, and are provided with extra gates, to be used when the tide rises above a certain height. Here there are two locks and a sluice, the larger of the two locks being capable of admitting a vessel 393 feet long and 59 feet wide, with a depth over sill of 28 feet 8 inches at ordinary low water. The walls of this lock are of brickwork, the exposed faces of which are of pressed bricks, and the rest of the work of ordinary hard burned bricks. In the case of the smaller lock an earthen slope supplies the place of the southern wall along the lock pond.

The Zuyder Zee locks at Schellingwoude have been named the "Orange Locks," after the heir apparent to the throne of Holland. Side by side in the long dyke, which is nearly a mile long, are three locks for ships, one sluicing lock, and three masonry channels for pumping out, by steam, the water when the tide in the Zuyder Zee is higher than the water in the canal. The largest ship lock is 315 feet long and 59 feet wide; the length of the other two is 239 feet, and their width 46 feet. These locks are built on a foundation of 8896 piles, and the dyke rests on mattrasses of fascines laid on the mud. The dyke is composed of sand and clay laid on fascines, and faced on the sea side with blocks of basalt and granite covered with clay. These locks have altogether twenty-seven pairs of gates, of which eleven pairs are of iron and sixteen of wood. The lock

walls are built exclusively of ordinary brickwork, in the same manner as those of the North Sea locks.

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It will thus be seen that, although the canal itself is shorter, the works on it are considerably more important than anything that occurs on the Suez Canal. The Orange Locks" were opened by the King of Holland in 1872, and the completed canal was opened by His Majesty on the 1st of November last, when he ordered that it should be known by the name of "The Port of Ymuiden." The total cost of this stupendous work has been about two millions sterling.

IV. ANIMAL GEOGRAPHY.*

NE of the most striking distinctions between the old and the new school of Natural History is the greatly increased amount of attention paid in the present day to the locality of every species. Our predecessors, if any specimen had not been derived from their own country, quietly dubbed it "exotic." Any creature from a tropical climate was labelled as a native of "the Indies,"-which might include either Venezuela, Hindustan, or New Guinea. To the modern naturalist, on the contrary, an accurate knowledge of the locality of every specimen he examines is a point of the first moment. Without this he regards it in much the same manner as a lawyer looks upon an unsigned document. "The structure, affinities, and habits of a spe cies now form only a part of its natural history. We require also to know its exact range at the present day and in pre-historic times, and to have some knowledge of its geological age, the place of its first appearance upon the globe, and of the various extinct forms most nearly allied

to it."

But though the correct locality of each species is now recorded in every systematic work on natural history, though local faunæ have been compiled, and attempts made at a general classification of the animal world from a geographical point of view, a work was still wanting which should com

The Geographical Distribution of Animals, with a Study of the Relations of Living and Extinct Faunas as elucidating the Past Changes of the Earth's Surface. By ALFRED RUSSEL WAllace. London: Macmillan and Co. Opening Address of the Biological Section of the British Association, 1876. By the President, ALFRED RUSSEL WALLACE.

bine and harmonise the mass of unconnected facts ascertained, and which should not merely propose an arrangement, but should demonstrate it by a careful and exhaustive analysis. This deficiency has been supplied by Mr. Wallace in a manner which must greatly enhance the well-merited esteem in which he is held by naturalists. The result is a work which in its department has no equal in any language, and which must at once be received as the text-book of zoological geography.

It may, at the first glance, appear an easy matter to determine the geographical distribution of the animal kingdom. We have only, it is said, to take a census of species in every country, to compare the returns, and to arrange our divisions accordingly; but the moment we make the attempt difficulties spring up on all sides. We require a trustworthy classification of animals, so that we may know what forms can be legitimately included under each species, genus, or family. We must then decide whether our classification is to be positive or negative, founded on the presence or on the mere absence of certain groups. Our own view, like that of Mr. Wallace, is that mere negative characteristics can have but very limited value. The extirpation of certain striking forms of life in a given island, whether effected by human agency or by natural causes, cannot give such island a higher rank as a zoological province than it had before. To distinguish two regions, a and b, we must be able to show that each contains something which is wanting in the other. Why, for instance, are the claims of Australia to rank as a distinct primary region so universally allowed? Not from the mere absence of monodelphic mammals, whether Carnivora, Rodentia, Ungulata, and the like, but because, in the stead of all these, there are didelphic groups which to some extent replace, or at least simulate, the monodelphic orders and families. This brings us to another fundamental principle, the higher the rank of the group present in one country and absent in another, the more fundamental is the distinction between them. Thus two adjacent islands might contain not a single Lepidopterous species in common; yet if all the species belonged to genera common to both islands we should rank both in the same region, sub-region, province, and district. But suppose that they had no genera or no families in common, we should consider it necessary to refer them at any rate to distinct sub-regions. If, again, the very orders are distinct, as is the case if we compare the mammals of Australia with those of the rest of the world,* we

* With the exception of the opossums of North and South America.

have before us a distinction of the highest order. But here is a fresh difficulty: it is only Australia which offers us so sharp a demarcation, and even this extends merely to the Mammalia: its birds and insects, though very distinct, not being separated from those of other parts of the world by so broad a boundary line. In separating region from region we cannot always avail ourselves of characteristics absolutely equal in value. This, as we shall afterwards see, has led some systematists to maintain that Australia and South America are marked off from each other and from the rest of the world by features more striking than those presented by any other region. We must therefore call in another principle, already shadowed forth in the admission that mere poverty of species cannot constitute a zoological region. We must take into consideration richness and variety of forms, as well as speciality. Nor must we insist upon being able to prove that all our primary divisions are of precisely equal rank. Nature will not adapt itself to our systematic classifications, whether geographical or morphological. Look, e. g., at our use of the term "order." It is applied equally to two such groups as Carnivora and Marsupialia. It must be admitted that the latter comprises at least four groups which, if more developed, might claim to rank as distinct orders. Or let us look at that vast assemblage of animated beings known as the "order" Coleoptera, but containing carnivorous, omnivorous, frugivorous, and lignivorous groups, differing widely as well in structure as in habits. Were they bulkier creatures, would not the 'stirps" Geodephaga be entitled to the position of an order equivalent and parallel to Carnivora? Thus we see that our morphological groups, as well as our geograpical regions, are by no means equal in value.

But to return: the question next arising concerns the foundation of our regional division. Shall it be founded upon the consideration of some one sub-kingdom or class, and, if so, upon which? Mr. Wallace, like some of his predecessors, takes the Mammalia as his standard, and only calls to his aid the distribution of other groups to determine doubtful points, or by way of corroboration. We cannot help thinking that insects have a higher claim to be selected for our guidance. They form, so to speak, the round numbers of the world's animal species, all other tribes and classes being in comparison a mere fractional amount : they are rarely purposely introduced by man into foreign countries, and the few which follow him parasitically, such VOL. VII. (N.S.)

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