In connection with these multifarious labors, M. de Mortillet published a considerable number of memoirs and of books of the highest order. He was a transformist from the very first, and performed all his various researches in the spirit of an evolutionist. His first publications were on conchology, and numerous memoirs between 1851 and 1862 related to subjects in that branch. During the same period he contributed many important works on the geology and mineralogy of Savoy. Among these were the History of the Land and Fresh-water Mollusks of Savoy and the Basin of Lake Leman, and a Guide to the Traveler in Savoy. His attention was afterward more entirely directed to prehistoric archæology and anthropology, and he published in 1866 a curious Study on the Sign of the Cross previous to Christianity. Of this period, too, are his Promenades, or Walks, in the Universal Exposition of 1867, and his Walks in the Museum of Saint-Germain, 1869. He founded, in 1864, the Recueil, or Collection of Materials for the Positive History of Man, which was afterward continued at Toulouse by M.E. Cartailhac. In 1879 he published a work on pottery marks—Potiérs allobroges, ou les Sigles figulins étudiés par les Méthodes de l'Histoire naturelle. In 1881, in co-operation with his son, Adrien de Mortillet, as artist, he published a magnificent illustrated work or album, Le Musée Préhistorique (The Prehistoric Museum); and in 1883, the volume Le Préhistorique (Prehistoric Archæology); two books which have taken rank as master works. A second edition of the Préhistorique appeared in 1885, and at the time of his death he was preparing a third, in which he was taking great pains to bring the matter up to the present condition of the science. Another important work was the Origines de la Chasse et de la Pêche (Origin of Hunting and Fishing). A considerable number of memoirs by M. de Mortillet appeared in various scientific journals, especially in the two founded by him—Les Matériaux pour l'Histoire primitive et naturelle de l'Homme, already mentioned, and L'Homme, which was established in 1884.
An epoch in M. de Mortillet's life was marked in 1873, when a discussion took place at the Anthropological Congress, in Lyons, between him and M. Abel Hovelacque concerning the precursors of man. The researches of the two masters had already led them, by a series of observations and deductions, to regard as certain the geological existence of a being intermediate between man and the monkey, which they called the Anthropopithecus, and they were trying to indicate, hypothetically, its leading characteristics.
M. de Mortillet's reasons for believing in the existence of this precursor of man as a definite being were presented in the Revue d'Anthropologie, in an article which was translated and published in the Popular Science Monthly for April, 1879. In this paper the author summarized the evidence, already copious, in favor of the existence of Quaternary man, and then took up the question, "Did there exist in the Tertiary age beings sufficiently intelligent to perform a part of the acts which are characteristic of man?" He then reviewed the researches of the Abbé Bourgeois at Thenay in the light of a collection of fire-marked flints which he had exhibited at the International Congress of Prehistoric Archæology and Anthropology held in Paris in 1867, and deduced from the result that "during the Middle Tertiary there existed a creature, precursor of man, an anthropopithecus, which was acquainted with fire, and could make use of it for splitting flints. It also was able to trim the flint flakes thus produced, and to convert them into tools. This curious and interesting discovery for a long time stood alone, and arguments were even drawn from its isolated position to favor the rejection of it. Fortunately, another French observer, M.J.B. Rames, has found in the vicinity of Aurillac (Cantal), in the strata of the upper part of the Middle Tertiary—here, too, in company with mastodons and dinotheriums, though of more recent species than those of Thenay—flints which also have been redressed intentionally. In this case, however, the flints are no longer split by fire, but by tapping. It is something more than a continuation, it is a development. Among the few specimens exhibited by M. Rames, whose discoveries are quite recent, is one which, had it been found on the surface of the ground, would never have been called in question." The evidence afforded by these flints was confirmed by a collection of flints from the Miocene and the Pliocene of the valley of the Tagus shown by Señor Ribeiro in the same exhibition, a considerable proportion of which bore evidence of intentional chipping.
Bearing upon this point was a chart of the Palæolithic Age in Gaul, drawn up by M. de Mortillet in 1871, and published in the Bulletin de la Société d'Anthropologie de Paris—"the only work of the kind extant"—in which were recorded five localities in which occurred supposed traces of man in the Tertiary, forty-one alluvial deposits in the Quaternary yielding human bones and industrial remains, and two hundred and seventy-eight caverns containing Quaternary fauna with traces of prehistoric man.
M. de Mortillet gave in another form his view of the sort of creature the hypothetical anthropopithecus should be in a paper on Tertiary Man, read before the Anthropological Section of the French Association for the Advancement of Science in 1885, when he said the question was not to find whether man already existed in the Tertiary epoch as he exists at the present day. Animals varied from one geological epoch to another, and the higher the animals the greater was the variation. It was to be inferred, therefore, that man would vary more rapidly than the other mammals. The problem was to discover in the Tertiary period an ancestral form of man a predecessor of the man of historical times. There were, he affirmed, unquestionably in the Tertiary strata objects which implied the existence of an intelligent being—animals less intelligent than existing man, but much more intelligent than existing apes. While the skeleton of this ancestral form of man had not yet been discovered, he had made himself known to us in the clearest manner by his works. The general opinion of the meeting after hearing M. de Mortillet's paper is said to have been that there could be no longer any doubt of the existence of the supposed ancestral form of man in the Tertiary period.
The discovery in Java, announced by Dr. Dubois, in 1896, of fossil remains presenting structural characteristics between those of man and those of the monkey, to which the name Pithecanthropus erectus was given, were accepted with hardly a question by M. de Mortillet and his colleagues as confirming his views.
At a banquet given to M. de Mortillet, May 1, 1884, by a number of anthropologists, when his portrait was presented to him, the hall was decorated for the occasion with a life-size picture of an ancient Gaul, executed according to his latest researches. The man was represented as having no hair on his body; with very long arms and very powerful muscles; his feet capable of being used in climbing trees, but with toes not opposable; his jaw strongly prognathous, but not at all equal to that of an anthropoid ape; his breadth strongly compressed laterally and his abdomen prominent; the skin not negroid, but of our present color; and the expression of his face was about as intelligent as that of an Australian.
In his Le Préhistorique M. de Mortillet attempted to determine how far distant was the epoch when Homo sapiens first appeared on the earth, by estimating the rate of progression of blocks which were carried by former ice fields, as he had observed them in Switzerland with Agassiz. His conclusion was that more than two hundred thousand years had elapsed since that event.
In 1894 M. de Mortillet proposed in the Société d'Anthropologie an important reform in chronology. Pointing out the inconvenience of using several different eras, such as the Foundation of Rome, the Flight of Mohammed, and the Proclamation of the French Republic, he suggested that ten thousand years before the Christian era be adopted as a general starting point. This would include all Egyptian chronology as known at the present day, and would leave five thousand years at the disposal of future discoverers.