on
was not strikingly apparent were merely described without any attempt
to relate them with each other, or with the other facts. A hypothesis
which left a great number of facts without explanation was necessarily
insufficient. The descriptions, in spite of all their individual
interest, did not constitute a homogeneous whole, a science. They were
merely a collection of more or less natural histories.

[1] See, for example, Reaumur, _Memoires pour l'histoire
des Insectes_, t. i., pp. 23-25.

Science only begins on the day when we have found the simple theory
which binds together all the facts at that time known, without of
course prejudicing the future. As the number of acquired facts
increases, if the theory in question continues to explain the new as
it explained the old, the science becomes more firmly established. If
we can imagine a time arriving when all the possible phenomena are
known, and the existing hypothesis still explains them, nothing
henceforth can overturn it, the science is completed. That is the
simple case in which a theory has been victorious; but if it is
contradicted by a single well-authenticated fact it must fall or
become modified. The more things a theory explains in the present the
more chance it has of success in the future. It is still only a matter
of chances, for the theory is always at the mercy of unforeseen
observation, which may rudely overthrow it.

There is no theory which must not be modified constantly, at least in
its details. To render it more and more general by successive
improvements is the aim to be pursued. A collection of studies
constitutes a science when a hypothesis has arisen already
sufficiently strong to oblige us to refer to it all new acquisitions,
and to compel us to see if they fortify or oppose it.

It would indeed be a narrow conception if we were to consider as
scientific the partisans of the theory alone; more than anywhere else
discussion is fruitful in the natural sciences; and if it is necessary
to be constantly preoccupied with the general ideas of the day, it is
not at all necessary to adhere to them servilely. The naturalists of
to-day are in possession of a formula with which we must always
preoccupy ourselves; in other words, there are natural sciences.

_The theory of Evolution._--This hypothesis which comes before all
others is the theory of evolution. This is not the place to expound
it, to go over the proofs which have been ama