If the Ancients had better intellects than ours, the brains of that age must have been better arranged, formed of firmer or more delicate fibres, fuller of “animal spirits.”  But if such a difference existed, Nature must have been more vigorous; and in that case the trees must have profited by that superior vigour and have been larger and finer.  The truth is that Nature has in her hands a certain paste which is always the same, which she is ever turning over and over again in a thousand ways, and of which she forms men, animals, and plants.  She has not formed Homer, Demosthenes, and Plato of a finer or better kneaded clay than our poets, orators, and philosophers.  Do not object that minds are not material.  They are connected by a material bond with the brain, and it is the quality of this material bond that determines intellectual differences.

But although natural processes do not change from age to age, they differ in their effects in different climates.  “It is certain that as a result of the reciprocal dependence which exists between all parts of the material world, differences of climate, which so clearly affect the life of plants, must also produce some effect on human brains.”  May it not be said then that, in consequence of climatic conditions, ancient Greece and Rome produced men of mental qualities different from those which could be produced in France?  Oranges grow easily in Italy; it is more difficult to cultivate them in France.  Fontenelle replies that art and cultivation exert a much greater influence on human brains than on the soil; ideas can be transported more easily from one country to another than plants; and as a consequence of commerce and mutual influence, peoples do not retain the original mental peculiarities due to climate.  This may not be true of the extreme climates in the torrid and glacial zones, but in the temperate zone we may discount entirely climatic influence.  The climates of Greece and Italy and that of France are too similar to cause any sensible difference between the Greeks or Latins and the French.

Saint Sorlin and Perrault had argued directly from the permanence of vigour in lions or trees to the permanence of vigour in man.  If trees are the same as ever, brains must also be the same.  But what about the minor premiss?  Who knows that trees are precisely the same?  It is an indemonstrable assumption that oaks and beeches in the days of Socrates and Cicero were not slightly better trees than the oaks and beeches of to-day.  Fontenelle saw the weakness of this reasoning.  He saw that it was necessary to prove that the trees, no less than human brains, have not degenerated.  But his a priori proof is simply a statement of the Cartesian principle of the stability of natural processes, which he put in a thoroughly unscientific form.  The stability of the laws of nature is a necessary hypothesis, without which science would be impossible.  But here it was put to an illegitimate use.  For it means that, given precisely the same conditions, the same physical phenomena will occur.  Fontenelle therefore was bound to show that conditions had not altered in such a way as to cause changes in the quality of nature’s organic productions.  He did not do this.  He did not take into consideration, for instance, that climatic conditions may vary from age to age as well as from country to country.