Lecture 17 — The duration and change of rivers.

The same parts of the earth are not always moist or dry, but they change according as rivers come into existence and dry up. And so the relation of land to sea changes too and a place does not always remain land or sea throughout all time, but where there was dry land there comes to be sea, and where there is now sea, there one day comes to be dry land. But we must suppose these changes to follow some order and cycle.

The principle and cause of these changes is that the interior of the earth grows and decays, like the bodies of plants and animals. Only in the case of these latter the process does not go on by parts, but each of them necessarily grows or decays as a whole, whereas it does go on by parts in the case of the earth. Here the causes are cold and heat, which increase and diminish on account of the sun and its course. It is owing to them that the parts of the earth come to have a different character, that some parts remain moist for a certain time, and then dry up and grow old, while other parts in their turn are filled with life and moisture. Now when places become drier the springs necessarily give out, and when this happens the rivers first decrease in size and then finally become dry; and when rivers change and disappear in one part and come into existence correspondingly in another, the sea must needs be affected. If the sea was once pushed out by rivers and encroached upon the land anywhere, it necessarily leaves that place dry when it recedes; again, if the dry land has encroached on the sea at all by a process of silting set up by the rivers when at their full, the time must come when this place will be flooded again.

But the whole vital process of the earth takes place so gradually and in periods of time which are so immense compared with the length of our life, that these changes are not observed, and before their course can be recorded from beginning to end whole nations perish and are destroyed. Of such destructions the most utter and sudden are due to wars; but pestilence or famine cause them too. Famines, again, are either sudden and severe or else gradual. In the latter case the disappearance of a nation is not noticed because some leave the country while others remain; and this goes on until the land is unable to maintain any inhabitants at all. So a long period of time is likely to elapse from the first departure to the last, and no one remembers and the lapse of time destroys all record even before the last inhabitants have disappeared. In the same way a nation must be supposed to lose account of the time when it first settled in a land that was changing from a marshy and watery state and becoming dry. Here, too, the change is gradual and lasts a long time and men do not remember who came first, or when, or what the land was like when they came. This has been the case with Egypt. Here it is obvious that the land is continually getting drier and that the whole country is a deposit of the river Nile. But because the neighboring peoples settled in the land gradually as the marshes dried, the lapse of time has hidden the beginning of the process. However, all the mouths of the Nile, with the single exception of that at Canopus, are obviously artificial and not natural. And Egypt was nothing more than what is called Thebes, as Homer, too, shows, modern though he is in relation to such changes. For Thebes is the place that he mentions; which implies that Memphis did not yet exist, or at any rate was not as important as it is now. That this should be so is natural, since the lower land came to be inhabited later than that which lay higher. For the parts that lie nearer to the place where the river is depositing the silt are necessarily marshy for a longer time since the water always lies most in the newly formed land. But in time this land changes its character, and in its turn enjoys a period of prosperity. For these places dry up and come to be in good condition while the places that were formerly well-tempered some day grow excessively dry and deteriorate. This happened to the land of Argos and Mycenae in Greece. In the time of the Trojan wars the Argive land was marshy and could only support a small population, whereas the land of Mycenae was in good condition (and for this reason Mycenae was the superior). But now the opposite is the case, for the reason we have mentioned: the land of Mycenae has become completely dry and barren, while the Argive land that was formerly barren owing to the water has now become fruitful. Now the same process that has taken place in this small district must be supposed to be going on over whole countries and on a large scale.

Men whose outlook is narrow suppose the cause of such events to be change in the universe, in the sense of a coming to be of the world as a whole. Hence they say that the sea being dried up and is growing less, because this is observed to have happened in more places now than formerly. But this is only partially true. It is true that many places are now dry, that formerly were covered with water. But the opposite is true too: for if they look they will find that there are many places where the sea has invaded the land. But we must not suppose that the cause of this is that the world is in process of becoming. For it is absurd to make the universe to be in process because of small and trifling changes, when the bulk and size of the earth are surely as nothing in comparison with the whole world.

Rather we must take the cause of all these changes to be that, just as winter occurs in the seasons of the year, so in determined periods there comes a great winter of a great year and with it excess of rain. But this excess does not always occur in the same place. The deluge in the time of Deucalion, for instance, took place chiefly in the Greek world and in it especially about ancient Hellas, the country about Dodona and the Achelous, a river which has often changed its course. Here the Selli dwelt and those who were formerly called Graeci and now Hellenes.

When, therefore, such an excess of rain occurs we must suppose that it suffices for a long time. We have seen that some say that the size of the subterranean cavities is what makes some rivers perennial and others not, whereas we maintain that the size of the mountains is the cause, and their density and coldness; for great, dense, and cold mountains catch and keep and create most water: whereas if the mountains that overhang the sources of rivers are small or porous and stony and clayey, these rivers run dry earlier. We must recognize the same kind of thing in this case too. Where such abundance of rain falls in the great winter it tends to make the moisture of those places almost everlasting. But as time goes on places of the latter type dry up more, while those of the former, moist type, do so less: until at last the beginning of the same cycle returns. Since there is necessarily some change in the whole world, but not in the way of coming into existence or perishing (for the universe is permanent), it must be, as we say, that the same places are not for ever moist through the presence of sea and rivers, nor for ever dry.

And the facts prove this. The whole land of the Egyptians, whom we take to be the most ancient of men, has evidently gradually come into existence and been produced by the river. This is clear from an observation of the country, and the facts about the Red Sea suffice to prove it too. One of their kings tried to make a canal to it (for it would have been of no little advantage to them for the whole region to have become navigable; Sesostris is said to have been the first of the ancient kings to try), but he found that the sea was higher than the land. So he first, and Darius afterwards, stopped making the canal, lest the sea should mix with the river water and spoil it. So it is clear that all this part was once unbroken sea. For the same reason Libya—the country of Ammon—is, strangely enough, lower and hollower than the land to the seaward of it. For it is clear that a barrier of silt was formed and after it lakes and dry land, but in course of time the water that was left behind in the lakes dried up and is now all gone. Again the silting up of the lake Maeotis by the rivers has advanced so much that the limit to the size of the ships which can now sail into it to trade is much lower than it was sixty years ago. Hence it is easy to infer that it, too, like most lakes, was originally produced by the rivers and that it must end by drying up entirely. Again, this process of silting up causes a continuous current through the Bosporus; and in this case we can directly observe the nature of the process. Whenever the current from the Asiatic shore threw up a sandbank, there first formed a small lake behind it. Later it dried up and a second sandbank formed in front of the first and a second lake. This process went on uniformly and without interruption. Now when this has been repeated often enough, in the course of time the strait must become like a river, and in the end the river itself must dry up.

So it is clear, since there will be no end to time and the world is eternal, that neither the Tanais nor the Nile has always been flowing, but that the region whence they flow was once dry: for their effect may be fulfilled, but time cannot. And this will be equally true of all other rivers. But if rivers come into existence and perish and the same parts of the earth were not always moist, the sea must needs change correspondingly. And if the sea is always advancing in one place and receding in another it is clear that the same parts of the whole earth are not always either sea or land, but that all this changes in course of time.

So we have explained that the same parts of the earth are not always land or sea and why that is so: and also why some rivers are perennial and others not.

131. After showing the cause of the generation of rivers, the Philosopher here determines about their duration. About this he does two things.

First, he presents his own opinion (351a19) and says that the same parts of the earth are not always moist or dry, but they vary according as rivers are newly formed or decay. As a result, changes occur affecting the earth, so that what is now dry, later becomes a sea and vice versa; and sea or dry land are not always present in the same part of the earth. But this takes place, not by chance, but according to a certain order and according to a certain turning of the heavens, just as all changes taking place in these lower regions are regulated according to the movement of the heaven.

132. second at the principle and cause of these changes (351a26), he explains what he has said. About this he does two things:

first, he assigns one cause of what has been said, at (352a17);

second, he rejects a cause held by others, at men whose outlook is narrow (352a17).

About the first he does two things:

first, he assigns the cause of the transmutation just mentioned, at (352a17);

second, he explains why such transmutations escape our notice, at but the whole vital process of the earth (351b8).

He says therefore first (351a26), that the cause and source of the above-mentioned transmutation is that the energy of the earth has its own kind of coming to a stop and old age, just as do bodies of animals and plants. But there is this difference: animals and plants suffer a stopping and old age, not successively with respect to different parts, but all at one time with respect to the whole, while, in the earth, this change is part by part, depending on the influence of heat and cold—for one part increases in heat or coldness, while another decreases, depending on the course of the sun and on other cycles of the heavenly bodies. Hence, depending on their differing position as to the aspects toward the sun and stars, the parts of the earth receive diverse power. As a result, some parts of the earth can continue being moist and watery for a long time according to a definite period—and this is, as it were, their youth or full state; later, they dry up and exist in what is for the earth old age, which naturally is accompanied by dryness, because of the absence of moisture. While these portions of the earth are drying up, other regions of earth are being vivified and become in part watery. This is indicated by the fact that in the spring of the year all things become, so to speak, young again by moisture; but in winter the same things grow old because of the excessive dryness. Moreover, in spring our part of the earth is alive, while elsewhere everything is already dried up. From this it is plain that old age and youth do not affect the earth as a whole (as animals and plants are affected) but one part at a time.

According to this, therefore, in certain areas of the earth, the springs that are dried up in this way cease to function. As a result, rivers that were once large become small, and finally dry up entirely; because the springs from which they originated are dry. Thus, in one part of the earth now grown old, the springs are dry, but in another part that has become watery, a corresponding number of new springs and rivers arise. Hence, because of these changes which affect streams, so that in one portion of the earth they perish and in another portion new ones arise, the sea too is changed as a consequence. Once the rivers dry up, the sea falls back and dry land appears where the sea was once swollen by the overflow of rivers. But where the sea has been dried up by accretions of earth deposited by rivers, it again is under water when a quantity of water has gathered.

133. second at but the whole vital process of the earth (351b8), he explains why these transmutations escape our notice, and says that these transmutations of sea and dry land escape us, because every natural change takes place not all at once but bit by bit—and the transmutations under discussion, which affect large sectors of the earth occur over long periods of time; and all nations die and pass away before a memory remains of such a change from its beginning to its end. For if the same peoples always remained in the same areas of the earth, some remembrance, even of things most ancient and of transmutations, could remain—but when one people is destroyed and a new one inhabits their land, no memory of the antiquities of the first remains among the second, much less among the third and fourth. Now the disappearances of nations closest to our times are brought on by wars, but some also by diseases and epidemics, and some by sterility: of these destructions some occur all at once on a large scale, and some gradually. Furthermore, the migrations of peoples from place to place go unnoticed, because some forsake a region from the start because the earth is becoming barren or weak, or because of wars; while others remain as long as they can find sustenance there. Hence, the time between the first departure and the last is sometimes long, and there is no memory of the first migration, even if the men do not die, but migrate. And as it is with forsaking lands, so with their settling: because of the length of time, there is no record when and by which races the land first began to be settled and when it was first changed from swamp into dry land and made habitable—this taking place little by little and over long periods of time. And he gives the example of Egypt, whose land was gradually drained dry, as though by the river. He mentions other lands, too, that have been transmuted and drained dry of moisture, and vice versa, as is plain in the text.

134. Then when he says, men whose outlook is narrow (352a17) he dismisses an explanation that some have suggested. About this he does three things:

first, he dismisses the false explanation and says that some, by looking at some slight thing, want to make a judgment about the entire heaven: for they think that the cause of such transmutation is that the whole world is changing in the sense that the heaven and the world are being newly formed. They say that the sea gets smaller, because it was beginning to be dried up by the sun from the very beginning: that is why many places formerly moist are now dry. But this is partly true, partly not. For it is true that certain places formerly tilled with water are now dried out; but the opposite is also true, because in some places the sea is found to have invaded areas that were formerly dry land. But it is false that the cause of this change is due to a generation of the earth. For it is ridiculous to postulate transmutations in the whole because of transmutations in small areas. Moreover, the size of the earth is almost as nothing compared with the heaven as a whole, for it takes on the status of a point.

135. second at rather we must take the cause of all these changes (352a28), he restates the true cause, and says that the true cause of those transmutations is this: just as a year is divided by different seasons, namely, by winter and summer and the customary seasons, so also a certain great cycle is divided according to established sequences by a great winter, wherein there is an excess of storms, and a great summer, in which there is excessive dryness; but not in the sense that these great excesses of storms and dryness occur at the same time over the entire earth or always over the same regions: they occur in different regions. And he gives the example of the deluge in one definite region of Greece in the time of Deucalion.

136. third at when, therefore, such an excess of rain occurs (352b3), from the foregoing he assigns a cause why rivers last, and says that when excessive storms occur in some region, the earth is saturated with enough moisture to generate rivers for a long time. This is a detail common to various explanations: whether the perpetuity of rivers is explained by large subterranean depths containing much water, as claimed by some, as cited earlier; or whether, according to our own opinion already given, the cause of their perpetuity is the size and thickness and coldness of mountains, by which they can receive and contain vast stores of water as well as generate it. But those places where the substance of the mountains is small and they are not very high, or where they are, so to speak, spongy [porous] so that moisture cannot be preserved in them, and are rocky, so as not to be able to receive water, and are clayey, so as not to be able to generate it, in such places, I say, the flow of rivers diminishes until the region again becomes moist. Therefore, one must believe that in whatever places an abundance of water falls during a great winter, the moisture of such places will be more perpetual, i.e., long-lasting. Yet, as time goes on, such places dry up and some become less moist until the return again of that period in which there is an excess of rain.

And so he concludes finally that, since change must take place in the whole universe (but not in the sense that the whole comes into being and is destroyed), if the world as a whole is perpetual, then it is necessary, as was said, that the same places be not forever moist, because of the sea or rivers, or dry, either—but places that previously were wet, become dry, and vice versa.

137. Then when he says, and the facts prove this (352b19), he manifests what he has said with examples. About this he does three things.

First, he gives three examples.

The first of which concerns the land of Egypt, which is found to be below the surrounding sea; for this reason certain of their kings were hindered from linking the two seas, when they saw that this would destroy the flow of the river water.

The second example concerns Lake Maeotis [Sea of Azov], in which the river flow produces so much silt that in his time it could only take boats much smaller than 60 years earlier.

The third example concerns the Bosphorus, which divides Europe from Asia. This strait is found smaller, and always tending to narrow, for the same reason.

138. second at so it is clear (353a14), he draws the conclusion that was chiefly intended and says that since there is no end to time and since the entire universe is eternal (according to his opinion presented in the Physics and in On the Heavens) it follows that neither the Tanais [Don] nor the Nile, which are very large rivers, have always been flowing, but the place whence they flow was once dry: because their work, i.e., their flow, has a limit. The same is true of other rivers. And if this is true of rivers, it must be true also of the sea into which rivers flow: that is why, at different times, sea and dry land are reversed. However, his supposition that the world and time are eternal is erroneous and opposed to the faith; nor are the arguments used to support these suppositions, demonstrations, as was shown elsewhere.

third at so it is clear (353a25), he summarizes what he has said, and the text is clear.