of species which we behold are the feeble remnants of the pristine plasticity and an exhausted force.[XII-2] This force of variation or origination of forms has acted rhythmically or intermittently, because each movement was the result of the rupture of an equilibrium, the liberation of a force which till then was retained in a potential state by some opposing force or obstacle, overcoming which it passes to a new equilibrium and so on Hence alternations of dynamic activity and static repose, of origination of species and types, alternated with periods of stability or fixity.  The timepiece does not run down regularly, but “la force procede par saccades; et . . . par pulsations d’autant plus energiques que la nature etait plus pres de son commencement.”

Such is the hypothesis.  For a theory of evolution, this is singularly unlike Darwin’s in most respects, and particularly in the kind of causes invoked and speculations indulged in.  But we are not here to comment upon it beyond the particular point under consideration, namely, its doctrine of the inherently limited duration of species.  This comes, it will be noticed, as a deduction from the modern physical doctrine of the equivalence of force.  The reasoning is ingenious, but, if we mistake not, fallacious.

To call that “evolutive force” which produces the change of one kind of plant or animal into another, is simple and easy, but of little help by way of explanation.  To homologize it with physical force, as M. Naudin’s argument requires, is indeed a step, and a hardy one; but it quite invalidates the argument.  For, if the “evolutive force” is a part of the physical force of the universe, of which, as he reminds us, the sum is fixed and the tendency is toward a stable equilibrium in which all change is to end, then this evolutive was derived from the physical force; and why not still derivable from it?  What is to prevent its replenishment in vegetation, pari passu with that great operation in which physical force is stored up in vegetable organisms, and by the expenditure or transformation of which their work, and that of all animals, is carried on?  Whatever be the cause (if any there be) which determines the decadence and death of species, one cannot well believe that it is a consequence of a diminution of their proper force by plant-development and division; for instance, that the sum of what is called vital force in a full-grown tree is not greater, instead of less, than that in the seeding, and in the grove greater than in the single parental tree.  This power, if it be properly a force, is doubtless as truly derived from the sunbeam as is the power which the plant and animal expend in work.  Here, then, is a source of replenishment as lasting as the sun itself, and a ground—­so far as a supply of force is concerned—­for indefinite duration.  For all that any one can mean by the indefinite existence of species is, that they may (for all that yet appears) continue while the external conditions of their being or well-being continue.