Sept 28, 1876] 



NATURE 



495 



we have examined, the temperature is iisually a little above the 

 freezing-point, Down in the valleys it sinks to perhaps pretty 

 near the freezing-point in some places, and in some very few 

 places it sinks a little below it, but it is only in one or two places 

 in the Atlantic and Pacific we find such extremely cold water. 

 Over the elevations the temperature is somewhat higher ; but 

 in the Atlantic and the Pacific, as a rule, the rise of temperature 

 on the ordinary elevations of the bottom of the sea is not above 

 two or three degrees. The temperature of the bottom of the 

 sea is, therefore, as a rule, a little above the freezing-point. 

 When we examine the temperature of such an ocean as the 

 Atlantic, from the surface down to the bottom, we find that it 

 gradually falls. On the surface its height is according to the 

 season of the year, according to the latitude, and according to 

 the heat of the sun at the locality observed, or at that from which 

 the surface-water is immediately derived. The temperature often 

 rapidly falls for a certain distance and then it more gradually 

 falls to a depth of about 500 fathoms, when it has a tem- 

 perature of something like 45°. That is a very general tem- 

 perature for a depth of about 500 fathoms. From that point 

 downwards the temperature very slowly and gradually falls, and 

 it falls till it reaches a temperature of 37° or 34", or, as I 

 have said before, sometimes below the freezing-point. Now, 

 the consequence of this is that we have a very uniform as well 

 as a very low temperature at the bottom of the sea, and we 

 shall see shortly the result of this on the distribution of animal life. 

 It is a uniform temperature, but it is a temperature which varies 

 within certain limits. The question comes — Whence does the 

 ocean derive this peculiar temperature ? aad this is a question of 

 very great difficulty, and one which I have not to-night sufficient 

 time to go into in detail, but I shall merely give you a general 

 idea of the impression which is on our minds after the observa- 

 tions of the Challenger with regard to the sources of temperature 

 in the Atlantic. The surface, as I have said, is affected by the 

 heat of the sun, and by the conditions of the latitude down to per- 

 haps about 500 fathoms. It is also very greatly affected by currents 

 which are moving through the sea, and which are mixing water 

 of different temperatures, and bringing water of different tempe- 

 ratures from different places. There is one set of currents which 

 is particularly marked and which tends to spread warmth over 

 the surface of the northern and southern seas, and modify the ocean 

 temperatures. These are the great currents which are running from 

 east to west driven by the ti'ade winds blowing along the equa- 

 torial region and driving before them the equatorial water. They 

 are met by the great continents — one is met by Cape San Roque 

 in South America, in the Atlantic, and against Cape San Roque 

 it divides : one portion going northward and another southward. 

 In the Pacific the current is met by the coast of Asia, and in the 

 same way one portion runs northwards and the other southwards. 

 Thus warm water, being driven to the north and south, becomes 

 mixed with colder water, and the temperature is modified and 

 ameliorated by it. It is likewise affected by other currents which 

 are produced by various reflections against coasts and other 

 obstacles. In this way we have water moving about on the 

 surface and conveying temperature from one place to another, 

 and rendering the temperature of these upper 500 fathoms ex- 

 tremely irregular. In the Atlantic we find that from this point — 



bout 500 fathoms — to the bottom the temperature steadily de- 

 creases until it comes down to near the freezing-point, no matter 

 the surface-temperature or the latitude. We have come to the 



onclusion that this great mass of water is moving from the 

 Southern Sea, and there seems to me to be very little doubt — 

 although this matter will require to be gone into carefully— that 

 the reason why this water is moving from the Southern Sea in 

 a body in this way is that there is a greater amount of evapora- 

 tion in the North Atlantic and over the Northern Hemisphere 

 generally, than there is of precipitation, whereas it seems almost 

 obvious that in the Southern Hemisphere, in the huge band of 

 low barometric pressure round the South Pole, the precipitation 

 is in excess of the evaporation. This is an extremely simple way 

 of accounting for this mass of cold water which it has been 



liitherto found impossible to account for on any reasonable 



heory. 



There is a minor phenomenon connected with this grand 

 system of circulation which passes partly through the atmosphere 



md partly through the ocean, which is extremely pretty, and of 



:hi5 1 will endeavour to give you a single illustration ; and in 



rder to understand it fully I will ask you to imagine for a moment 

 terrestrial globe and the relations in volume and position which 



he oceans and the contuients bear to one another. You remember 



the vast accumulation of water rovmd the South Pole, and in the 

 South Pacific ; and the '* land hemisphere " almost ih the centre 

 of which we now stand, with the two great gulfs, the Pacific and 

 the Atlantic running up into it, almost cut off by land and shallow 

 water to the northern end, but opening widely to the Southern 

 Sea. Now imagine the depth along a line joining Cape Horn 

 with the Cape of Good Hope to be 3,000 fathoms, the bottom 

 temperature being 30', and the temperature at 2, 500 fathoms 32°, 

 and suppose a continuous barrier to extend between the two 

 capes to the north of this line, rising 500 fathoms from the 

 bottom ; it is clear that if the movement of the mass of cold 

 bottom water be constantly from the south to the north, no water 

 colder than 32° can ever enter the Atlantic, and however deep 

 portions of that ocean may be, water under that temperature can 

 never be found in it to the north of the 2, 500 fathom barrier. 



Although this is an imaginary cise, at least one which is 

 scarcely in nature so simple as I have represented it, we find the 

 same law acting perpetually. In various parts of the world there 

 are little isolated seas, and circumscribed basins of the great 

 ocean, surrounded by sach barriers, and we can tell at once the 

 height of the lowest part of the barrier by the temperature of the 

 bottom-water of the basin, for we know that it must correspond 

 with the depth at which the like temperature occurs in the outer 

 ocean from which the basin derives its supply. 



I have now only a few minutes left to refer to the last of the 

 three questions selected for consideration, the distribution and 

 nature of the deep-sea fauna. The deep-sea is by no means 

 barren, but on the contrary a fauna very remarkably constituted 

 and comparatively rich, is universally distributed even to the 

 greatest depths. It was our impression that when we examined 

 this fauna we should find it very analogous to that of the ancient 

 chalk, for we believed, and we believe still, that the deposition 

 of chalk has been going on continuously in various parts of the 

 ocean, from the chalk period to the present time. In this expec- 

 tation we were to a certain extent disappointed, for the species 

 found in the modern beds are certainly in very few instance? 

 identical with those of the chalk or even with those of the older 

 tertiaries. But although the species, as we usually regard spacies, 

 are not identical, the general character of the assemblage of 

 animals is much more nearly allied to the cretaceous than to any 

 recent fauna. You have in the Clyde district some extremely 

 interesting little localities — one for instance in Loch Fyne near 

 Inverary, and another at Oban — where animals are found in 

 shallow water which are usually only found in deep water, and 

 other animals which are chiefly confined to the Arctic Seas. 

 Prof. Edward Forbes called these animals Boreal outliers, and 

 believed that the little tiasins in which they occur in this country 

 — which are always cherished dredging spots for naturalists — are 

 spots where, owing to the configuration at the bottom and to other 

 causes, patches of the old fauna have been entangled and retained 

 at the close of the glacial period. 



Here and there on the surface of the earth we seem to have, in 

 like manner, what we may call Abyssal outliers, spots where, 

 during some process of elevation, the abyssal fauna has been 

 caught and kept at an accessible depth. Such spots occur off 

 the coast of Japan, near Yokohama, at various places among the 

 Philippine Islands, off the coast of Portugal, and off the north 

 coast of Scotland, and from each of these strange and beautiful 

 things were brought to us from time to time, which seemed to 

 give us a ghmpse of the edge of some unfamiliar world. Among 

 these were the lovely and wonderful Euplectella!, and glass-rope 

 Hyaloneinas, and bird-nest-like Holtenias, and many others of the 

 hexradiate order of sponges, the representatives, and no doubt the 

 descendants, of the Vetitriculius of the old chalk ; and the grace- 

 ful sea-lilies belonging to the Pentacrinida, and the Apiocrinida, 

 whose aspect carries us back at once to the clays of the Liis and 

 the terraced limestones of the Jura. 



The fauna of the deep sea is wonderfully uniform throughout, 

 no one who has once seen it can fail to recognise]this general uni- 

 formity, whether he examines it in the middle of the Pacific, in 

 either trough of the Atlantic, or in the Southern Sea ; and yet, 

 although in different localities the species are evidently represen- 

 tatives, to a critical eye they are certainly not identical, and I 

 believe that one of the most important lines of inquiry which 

 have been opened up to us by these investigations is the range 

 and amount of variation, or possibly the passage of one apparent 

 species into another over this vast area, remoteness in space 

 being, when we consider the conditions of migration with the 

 accompanying change in surrounding circumstances, equivalent to 

 lapse of time. 



