But how comes it that the art of one age differs from that of another?  At first sight it seems odd that art, which is the expression of man’s sense of the significance of form, should vary even superficially from age to age.  Yet, deeply considered, it is as certain that superficially art will always be changing as that essentially it cannot change.  It seems that the ape-instinct in man is so strong that unless he were continually changing he would cease to create and merely imitate.  It is the old question of the artistic problem.  Only by setting himself new problems can the artist raise his powers to the white heat of creation.  The forms in which artists can express themselves are infinite, and their desire to express themselves keeps up a constant change and reaction in artistic form.  Not only is there something of the ancestral ape in man, there is something of the ancestral sheep; there are fashions in forms and colours and the relations of forms and colours; or, to put the matter more pleasantly, and more justly, there is sufficient accord in the sensibilities of an age to induce a certain similarity of forms.  It seems as though there were strange powers in the air from which no man can altogether escape; we call them by pet names—­“Movements,” “Forces,” “Tendencies,” “Influences,” “The Spirit of the Age”—­but we never understand them.  They are neither to be frightened nor cajoled by our airs of familiarity, which impress the public only.  They exist, however, and if they did not we should have to invent them; for how else are we to explain the fact that not only do the artists of a particular period affect particular kinds of form, but that even the spectators of each new generation seem to be born with sensibilities specially apt to be flattered by them.  In this age it is possible to take refuge under the magic word “Cezanne”; we can say that Cezanne has imposed his forms on Georgian painters and public, just as Wagner imposed his on Edwardian musicians and concert-goers.  This explanation seems to me inadequate; and in any case it will not account for the predominance of formal fashions in ages undominated by any masterful genius.  The spirit of an artistic age is, I suspect, a composition that defies complete analysis; the work of one great mind is generally one part of it, the monuments of some particular past age are often another.  Technical discoveries have sometimes led to artistic changes.  For instance, to men who have been in the habit of painting on wood, the invention of canvas would suggest all sorts of fascinating novelties.  Lastly, there is a continual change in the appearance of those familiar objects which are the raw material of most visual artists.  So, though the essential quality—­significance—­is constant, in the choice of forms there is perpetual change; and these changes seem to move in long flights or shorter jumps, so that we are able, with some precision, to lay our fingers on two points between which there is a certain amount of art possessing certain common characteristics.  That which lies between two such points historians call a period or movement.