By JOSEPH WALLACE.

Every now and then some weather sage predicts extremely cold winters, and another ventures to say that the sun is gradually losing heat and in time Arctic cold will prevail over the globe. Whatever may have been the changes during the vast cycles of time prior to the advent of man, or whatever may be the changes in the time to come, one thing is quite certain; that our climate has been much modified within the past two or three thousand years.

"There have been fifteen climatic changes since the beginning of the glacial age, each change lasting 10,500 years, and each change reversing the season in the two hemispheres, the pole which had enjoyed continuous summer being doomed to undergo perpetual winter for 10,500 years, and then passing to its former state for an equal term. The physical changes upon the earth's surface during the past 80,000 years modified the changes of climate even in the Arctic regions, so that the intense cold of the former epochs was much modified during the latter epochs." Reckoning these climatic changes in their order, we had entered the epoch of a more genial temperature about fifteen hundred years ago; and if no disturbing change takes place during the present epoch, we may reasonably expect a gradual modification of our winters for nine thousand years to come. The changes to intense cold from perpetual summer during the greater part of the glacial period are supposed to have been caused by the high temperature of the north pole as compared to that of the south pole, owing to the distribution of land around the two, the south having almost none.

Dr. Croll thinks it was caused by the varying inclination of the earth's axis, which produced the relative position of the two poles toward the sun to be periodically reversed at distant periods. Dr. James Geikie agrees with Croll on the reverse of seasons every 10,500 years during certain periods of high ellipticity of the earth's orbit.

But it may be asked, "How could the fauna and flora propagate themselves under such conditions?" The flora itself at the quaternary age was of extreme vigor. We know this from the little which is left us, but more especially from the presence of a large number of herbivorous animals - stags, horses, elephants, rhinoceros, etc. - which animated the plains and valleys of Europe and America at the same time. Evidently they could not have lived and propagated themselves without abundant vegetation for nourishment and development.

That which has deceived the adherents of the glacial theory, as understood in its absolute sense, is, they have generally placed a too high estimate on its extent and intensity. It needs but a little effort of the reasoning powers to come to the conclusion that the earth had cooled to the degree that all animal and vegetable life could exist upon it, and that a portion of the earth's surface permanently covered with snow and ice was absolutely indispensable to the existence, perpetuity, and well-being of animal and vegetable life. Again, they have attributed to the glaciers the rocks, gravels, and other material which they have found spread here and there long distances from the mountains. The transportation of the so-called erratic rocks has appeared inexplicable in any other way, and the piles of rock and gravel have been considered so many moraines, that is, deposits of diverse material transported by the glaciers. They do not regard the probability of other agents taking the place of glaciers, and undervalue the moving power of water. Water in liquid state has often produced analogous effects, and it has often been the error of the glacialists to confound the one with the other. The erratic rocks and the moraines are undoubtedly the ordinary indications of the ancient gravels, but, taken isolatedly, they are not sufficient proof. In order to convince they should be accompanied with a third indication, which is the presence of striated rocks which we find in the neighborhood of our actual glaciers. When all these signs are together then there is hardly a possibility of error, but one alone is not sufficient, because it can be the effect of another cause.

No doubt the temperature was really lower at the quaternary age and at the epoch generally assigned to man's advent in European countries, but the difference was not so great as some say. A lowering of four degrees is sufficient to explain the ancient extension of the glaciers. We can look on this figure as the maximum, for it is proved to-day that humanity played the main role in the glacial phenomena. The beds of rivers and the alluvia are there to tell that all the water was not in a solid state at that time, that the glaciers were much more extended than in our days, and that the courses of the rivers were infinitely more abundant. When this is understood we can reasonably reduce the extension of the ancient glaciers, the lowering of the temperature at the quaternary age, and account for the uninterrupted life of the fauna and flora. However, we must not fall into the opposite excess and assert, as some have done, that the glacial period is comparatively recent, the traces of which are too plain and fresh in some localities to assign to it an age prior to man, and that the temperature has rather lowered itself since this epoch. The ancient extension of the glaciers has been followed by a corresponding growth and extension of animal life, thus proving that the permanence of glaciers is a wise provision and absolutely essential to man and the high orders of animals and vegetation. The ancient extension does not prove alone that it was much colder than in historic times, for the animals themselves are proof of this.

At that time the plains of Europe, and of France in particular, were animated by herds of reindeer, gluttons, camels, and marmots, which one does not find to-day except in the higher latitudes or more considerable heights. The mammoth and rhinoceros are no exception to this, for naturalists know they were organized to live in cold countries.

Space will not permit us to pursue this point further, or speculate on the probable climatic conditions of the ice age; but we can carry ourselves back a few thousand years and describe the climate of Europe and neighboring countries of Africa and Asia. Herodotus describes the climate of Scythia in terms which would indicate in our day the countries of Lapland and Greenland. He shows us the country completely frozen during eight months of the year; the Black Sea frozen up so that it bore the heaviest loads; the region of the Danube buried under snow for eight months, and watered in summer by the abundant rains which gave to the river its violent course. The historian adds that the ass cannot live in Scythia on account of the extreme cold which reigns there. The following century Aristotle makes the same remarks concerning Gaul. His contemporary, Theophrastes, tells us that the olive tree did not succeed in Greece more than five hundred furlongs from the sea. We can assure ourselves that both the ass and the olive thrive in these countries at the present day.

Three centuries later, Caesar speaks frequently and emphatically of the rigor of winters and early setting in of cold in France, the abundance of snow and rain, and the number of lakes and marshes which became every moment serious obstacles to the army. He says he is careful not to undertake any expedition except in summer. Cicero, Varro, Possidonius, and Strabo insist equally on the rigor of the climate of Gaul, which allows neither the culture of the vine nor the olive. Diodorus of Sicily confirms this information: "The cold of the winters in Gaul is such that almost all the rivers freeze up and form natural bridges, over which numerous armies pass quite safely with teams and baggages; in order to hinder the passengers to slip out upon the ice and to render the marching more secure, they spread straw thereon."

Virgil and Ovid insist on the severity of cold in the regions of the Danube. The first describes the inhabitants of these miserable countries withdrawing themselves into caves dressed with the skins of wild beasts. Ovid, who had passed several years of his life in that region, is more precise in his description. He says the wine has changed itself here (Black Sea) into a solid frozen mass; one gives it to drink by pieces. Fearing of being accused of poetic exaggeration he appeals to the testimony of two ancient governors of Moesia, who could establish the facts like himself. The author who would give such accounts of the Black Sea in our days would risk his reputation for veracity.

Italy, too, experienced its part of the cold in early days. Virgil tells us of the snows being, heaped up, rivers which carried ice along, the sad winter which split the stone and bound up the course of large streams, and all this in the warmest part of Italy, at the base of the walls of Taranto. Heratius affirms that the Soracte, a neighboring mountain of Rome, was whitened with thick snow, rivers frozen, and the country covered with snow. To-day the snow stays very little upon the Soracte and never in the country around Rome. During the four or five centuries which followed, writers speak of the severity of climate in Northern Italy, the lagoons on the Adriatic being frozen over. Algiers was much colder then than now. The Danube, Rhine, and other rivers in Europe, the Nile in Africa, the Amazon in South America, the Mississippi and Missouri in North America, had quite different volumes two thousand years ago than their present actual ones, and they especially rolled much greater masses of water.

There is everything to show a modification of climate in our own days. If this goes on in the future as in the past, there will be a marked difference in the temperature two or three hundred years from now. Even a degree in a thousand years would effect a great change in the course of time. The lowering of four degrees established the ancient extension of glaciers, though it did not interrupt animal or vegetable life. Fifty-four of the fifty-seven species of Mollusca have outlived the glacial age, and all our savage animals - even a certain number which have disappeared - date equally from the quaternary, and were contemporary with the great extension of the glaciers. - Popular Science News.