the embryo and that of the complete
organism. The impetus which causes a living being to grow larger, to
develop and to age, is the same that has caused it to pass through the
phases of the embryonic life. The development of the embryo is a
perpetual change of form. Any one who attempts to note all its
successive aspects becomes lost in an infinity, as is inevitable in
dealing with a continuum. Life does but prolong this prenatal evolution.
The proof of this is that it is often impossible for us to say whether
we are dealing with an organism growing old or with an embryo continuing
to evolve; such is the case, for example, with the larvae of insects
and crustacea. On the other hand, in an organism such as our own, crises
like puberty or the menopause, in which the individual is completely
transformed, are quite comparable to changes in the course of larval or
embryonic life--yet they are part and parcel of the process of our
ageing. Although they occur at a definite age and within a time that may
be quite short, no one would maintain that they appear then _ex
abrupto_, from without, simply because a certain age is reached, just as
a legal right is granted to us on our one-and-twentieth birthday. It is
evident that a change like that of puberty is in course of preparation
at every instant from birth, and even before birth, and that the ageing
up to that crisis consists, in part at least, of this gradual
preparation. In short, what is properly vital in growing old is the
insensible, infinitely graduated, continuance of the change of form.
Now, this change is undoubtedly accompanied by phenomena of organic
destruction: to these, and to these alone, will a mechanistic
explanation of ageing be confined. It will note the facts of sclerosis,
the gradual accumulation of residual substances, the growing hypertrophy
of the protoplasm of the cell. But under these visible effects an inner
cause lies hidden. The evolution of the living being, like that of the
embryo, implies a continual recording of duration, a persistence of the
past in the present, and so an appearance, at least, of organic memory.

The present state of an unorganized body depends exclusively on what
happened at the previous instant; and likewise the position of the
material points of a system defined and isolated by science is
determined by the position of these same points at the moment
immediately before. In other words, the laws that govern unorganized
matter