Now, after all that we have said, this is how we may, in our opinion, represent by a graphic figure the whole of the vertebrate kingdom,[315] in the present and in the past. Let us image a conical figure: the individual-element of which we have spoken will occupy its summit. From this point a number of straight lines, few at first, will start, branching off and always multiplying themselves with more or less regularity, but so as to form an immense cone.[316] Each of these straight lines would represent a _specific modification_, accomplished after a certain number of generations under the combined influence of the ambient medium and of some considerable time: in other terms, each ramification would represent a species having once existed or now existing on our planet. The length of each line would measure the time which the species in question has existed. These lines would never converge, because we do not believe in the creation of permanent species by means of hybridity. Now the mind must admit here all possible combinations; certain species have disappeared without producing any others after them:--others exist actually without our having any idea of one of the intermediary species which have been allied to primitive species;--others have subsisted slightly or not at all altered from the remotest antiquity up to our own days, thus becoming through contemporaneous time the transformed descendants of fossil species, of which they were also formerly the contemporaries; it is even not impossible but that certain species succeeding one another may have presented a retrograde evolution, so that we must not always conclude that because one animal is only inferior to another, it has therefore preceded it:--without going so far as all this, the evolution of certain species may have presented a long time of cessation whilst all others were progressing around them, so that they appear to have retrograded. This is what has made M. Michelet[317] say, “Nature has not progressed with a continuous flow, but with retrograde movements, and stoppages, which allow her to harmonise everything.” These times of repose in a specific evolution, as well as the hypothesis of successive geneses which are already admitted, explain how the stratified beds of the earth’s surface, in showing from low to high what we may call more perfect organic means, unveil at the same time to our eyes here and there a certain number of species, inferior in organisation to those in the most ancient rocks.