The following essay is the 3rd part of the first essay in the book: The Beginnings of Buddhist Art and other essays in Indian and Central-Asian Archeology by A. Foucher, who herein discusses the important aniconic phase of Buddhist art, giving a reasonable thesis for its production and development. I have illustrated the essay with some files from Wikimedia. It was originally published in French in 1911, and later translated by L.A. Thomas and F.W. Thomas
For further texts and readings in English please follow up from THE SOURCE.
 Such is the sole part which hypothesis plays in our theory. The whole subsequent development of Buddhist art flows logically from these premises; and henceforth there are none of the still surviving documents which do not successively corroborate the various stages of its evolution. The oldest monuments which have come down to us from Indian antiquity are a few rectangular coins of copper or silver. Now it is very remarkable that, among the symbols with which they are punch-marked, the tree, the wheel and the stūpa play a considerable, and indeed, on many of them, a predominant part.  Thanks to the chance of their discovery, the existence of the signacula, which we imagined to have been made for the use of pilgrims, ceases to be, for as far back as we can go, a pure conjecture. Better still, we can clearly discern in the infantile simplicity of these emblems the style of the most ancient manifestations of the religious art of the Buddhists. They are, properly speaking, less images than hieroglyphics endowed for the initiated with a conventional value: and, at the same time, we succeed in explaining to ourselves what we have already more than once had occasion to note, that is, the abstract and quasi algebraical character of this art at its commencement. 
Buddha overcoming the Nāga (Sāñchi)
Moreover, we easily conceive that, in consequence of being conveyed beyond the great centres of pilgrimage,  artistic emblems of this sort may have seen their initial signification modified. They came, by degrees, to be regarded less as mementos of sacred spots than as figurative representations of miracles, the memory of which was connected with those places. In other words, in proportion as they were propagated further and further from their place of origin, their topographical and local character diminished more and more, to the advantage of their symbolical and universal value, until they ended by becoming the common patrimony of the image-makers and being fabricated everywhere without distinction where a Buddhist donor ordered them. It is just this state of diffusion and subsequent generalisation that is proved to us even in the IVth century by the banality and dispersion of the so-called “Buddhist” coins.
But we must hasten, in this rapid sketch, to come to the monuments whose Buddhist character can no longer be disputed. We know what impulse was towards the middle of the third century given by the imperial zeal of Aśoka to the religious foundations of the sect. It is, therefore, only the more curious to observe how, even a hundred years after him, the school of Central India continues to follow faithfully in the beaten track of the past. From this point of view, the four gates of Sāñchī, which we have had the good fortune to retain almost intact, may furnish a fairly safe criterion of the degree of persistence of the ancient usages.
Now Fergusson long ago remarked there the extreme frequency of what he called “the worship” of the tree, the stūpa and the wheel. According to statistics hardly open to suspicion, since they were drawn up in support of theories quite different from ours, the first emblem is repeated no less than 67 times, the second 32 times; and if the last does not reappear more than 6 times, this number suffices, nevertheless,  to assure it the third place in the order of importance of the subjects.  We have not, of course, to follow Fergusson in the strange anthropological speculations which he has engrafted on to these observations. All that we should be tempted at first to read in his table would be the preponderance of the miracle of the Sambodhi, or of the Parinirvāṇa, over that of the Dharmacakra-pravartana.
In reality the larger number of the first two symbols depends upon another cause. The artists proceeded to apply to the Buddhas of the past the formulas which had at first served for the Buddha of our age. People were pleased to level all the seven by representing them at one time by their funeral tumulus, at another, and much more frequently, by their empty throne under their Tree of Knowledge:  the wheel alone had remained the special apanage of our Śākya-muni, and consequently was repeated only at rarer intervals. But these are only subsidiary details; taking these figures all together, their imposing total testifies loudly enough to the constant repetition in traditional form of what we know, from the evidence of the coins, to have been the first attempts at Buddhist art.
Being forced to cover the relatively extensive surfaces placed at their disposal, the sculptors of  Sāñchī evidently commenced by re-editing profusely, right in the middle of the second century before our era, the summary and hieroglyphic compositions which they had inherited from their direct predecessors, the makers of religious objects in the fifth century.
- To quote only the latest study, cf. D. B. Spooner, A new find of punch-marked coins, in Arch. Survey of India, Annual Report 1905 – 1906, 1909, p. 150. According to the excellent analysis which Dr. Spooner has given of this discovery, out of 61 coins 22 bear all three symbols at once and 22 others associate the two last together. ↩
- Cf. for instance, Art Greco-bouddhique du Gandhāra, p. 608. ↩
- Cf, Fergusson, Tree and Serpent-Worship, 2nd edition, 1873, pp. 105 and 242. Here is the table, in which he has included the data of the sole gate of one of the small neighbouring stūpas:
Tree, Stūpa, Wheel.
South Gate 16 5 1
North Gate 19 8 2
East Gate 17 9 1
West Gate 15 10 2
Only Gate 9 6 4 ↩
- The decisive reason for the predominance of the inspired compositions of the type of Sambodhi over all the others will be given a little further on, p. 19. ↩