82 Content 5 Size 4 Symmetry 1 Rhythm II. Variations in Alternate Unit (and Principal Unit) 32 Content 11 Richness 9 Human figure 2 Rhythm 4 Symmetry 3 Different level 3 Disorder 5 Size 1 Symmetry 3 Rhythm 1 Disorder III. Variation in Distance 1 Disorder Several questions have been raised in this discussion of variations, but one which seems directly leading from it will be considered next. When is variation _necessary_ in a repeated series? We have considered the numerous cases where variation is _possible_, and the different ways in which a series may vary according to the idea to be expressed. Moreover, what appeared to be exceptions to the rule were shown to be guided by a desire for some other effect than repetitions as such. But when do we demand variation in a series? Is there any case where variation of the unit is not only allowable, but positively necessary to its æsthetic value? There were no experiments on this question, for it will be seen from what follows that they would have been impracticable. But observation of several thousand photographs has made the following clear: When the series consists of objects having an æsthetic significance of their own, not depending on something else for their value, then variation is demanded. In other words, when a thing is an end in itself, we do not tolerate an _exact_ duplicate. It may have a place in a series of others similar to it, but its own meaning loses force if another is beside it precisely alike. When, however, an object has no great significance by itself, or when however great its value, it be regarded as means to something greater, hence not an end in itself, it may be repeated without variation. This principle may be stated from another point of view: Any work of art, of the _highest_ significance in itself alone, must not be repeated at all. There must not be even the suggestion of repetition. The highest values are individual, and to have a copy or a series defeats its whole reason for being. Thus, a second Sistine Madonna, or a series of Venuses, would shock our whole æsthetic feeling. Moreover, we do not want a _suggestion_ of repetition; even a series of different Madonnas in similar frames would take away from the significance of each, in so far as they were regarded as a series, and not as a mere collection of detached units.