144 THE PROBABLE FUTURE OF THE HUMAN RACE. 



sand, one hundred thousand, or even of several hundred thousands of 

 years. It is still possible to foresee for these distant times certain ten- 

 dencies and certain conditions of the human species. We should, how- 

 ever, bear in mind that which renders doubtful even the most plausible 

 suppositions. For a lapse of time so prolonged, we cannot tell whether 

 some great terrestrial or cosmical event may not change absolutely 

 external conditions. Our globe may experience depressions and eleva- 

 tions which may alter entirely" the nature of the habitable surface. 

 New diseases may be manifested among men of which we have now no 

 idea. These maladies may destroy a whole race or even affect the 

 entire human species. Astronomers have shown us that the variations 

 of the plane of the ecliptic and of the eccentricity of the earth's orbit 

 are not of a nature to produce any sensible change in climates. In 

 stating this fact, however, Mr. Croll * is compelled to admit that the 

 accumulation of ice, the effect of the latter cause acting alternately 

 upon the north and upon the south pole, must produce changes in the 

 currents and in the dominant winds, which must be followed by changes 

 in climate in all parts of the world. There must have been several 

 glacial periods iu each hemisphere. The least ancient in the northern 

 hemisphere must have commenced about two hundred and forty thou- 

 sand and ended about eighty thousand years ago. Sir Charles Lyellt 

 has disputed the opinions of Mr. Croll, and according to him the perio- 

 dicity of the glacial periods must be very uncertain. Still we cannot 

 overlook the possibility of such events, the effect of which would be to 

 drive all organized beings from each polar region toward the temper- 

 ate zones, and those of the temperate regions to the equatorial, produc- 

 ing naturally the extinction of many species, and as regards man of 

 many races. 



Finally, who can foresee what may happen to our entire solar system? 

 It is moving with great rapidity in a certain direction. Perhaps some 

 time it may come into some part of the universe much warmer or colder 

 than the space it has passed through for several millions of years. The 

 sun, also, may change. Events such as these may destroy not only 

 man but all the organized beings of our globe. 



Setting aside these hypothetical cases, which are beyond the power of 

 science to foresee, let us pass to facts which are, on the contrary, abso- 

 lutely certain. 



The effect of the oxygen of the air, and the incessant action of human 

 labor, is diminishing the quantity of metals and of coal, accessible with- 

 out too much effort, on the surface of the earth. Undoubtedly, the 

 genius of savants will discover processes for working mines to greater 

 depths, and for profiting by the metallic oxides distributed through the 

 soil. New combustibles also may be discovered, but they can never be 

 as advantageous as those we now enjoy, and the metallic dust, as for 



* Croll, iu Edinburgh Philosophical Magazine, 1S67 and 1868. 

 i Sir Charles Lyell, Principles of Geology, latest edition. 



