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.

Perhaps, however, M. Naudin does not mean that "evolutive force," or the
force of vitality, is really homologous with common physical force, but
only something which may be likened to it. In that case the parallel has
only a metaphorical value, and the reason why variation must cease and
species die out is still to seek. In short, if that which continues the
series of individuals in propagation, whether like or unlike the parents,
be a force in the physical sense of the term, then there is abundant
provision in Nature for its indefinite replenishment. If, rather, it be a
part or phase of that something which directs and determines the
expenditure of force, then it is not subject to the laws of the latter, and