As to explaining how a part of the ancient species has been able to modify itself whilst another has remained stationary, we must admit that all these influences of _medium_ have always been exclusively local, so that all the coexisting vertebrata have never been able to submit at once to its influence. We must understand by medium, _the whole of the circumstances, past or present, which are able to influence organism mediately or immediately in any manner whatsoever_. The ancestors of an animal, as well as the sun which warms it, and the parasites which devour it, make up a part of this medium. But if it is easy to explain variety by the medium, it is a difficulty against which the mind struggles. How can we explain ascending and progressive variety? must we believe in some finality, an end settled beforehand? We do not think so. Finality is a sort of divine prevision, and the world as regards this hypothesis is still in tutelage; we would rather believe in a creating intelligence. A simple example will make our meaning understood. In the vegetable world this strikes us forcibly:--the most simply formed plants are precisely those which approach most nearly to animals by reason of their physiological manifestations.[318] The plants which they call superior, by placing them in an organographic point of view, are in reality inferior, so that these plants are simple in reference to the dicotyledons which have necessarily succeeded them, and there has been in reality a retrograde march of life, instead of the ascending march of the animal kingdom. Must we seek for the reason of this difference in the presence of a nervous system? We think so. We then would admit that organism would tend to modify itself by an inconscient act of the will, analogous to those which rule most physiological actions; this would be something like the possible increase or growth of the head by reason of the influence of civilisation of which we have before spoken.[319] And whilst all the specific varieties would result among plants from the influence of the physical medium, we must add to the notion of this medium, as regards animals, the nervous activity of the ancestors.