As regards economy, any principle of maximum economy, where many
functions have to be fulfilled, will, we may very safely predict,
involve as far as possible mutual helpfulness in the processes
going on. Thus the process of the development towards meeting any
particular external conditions, A, suppose, will, if possible,
tend to forward the development towards meeting conditions B; so
that, in short, where circumstances of morphology and physiology
are favourable, the ideally economical system will be attained
when in place of two separate processes, a, ß, the one process y,
cheaper than a + ß, suffices to advance development
simultaneously in both the directions A and B. The economy is as
obvious as that involved in "killing two birds with the one
stone"—if so crude a simile is permissible—and it is to be
expected that to foster such economy will be the tendency of
evolution in all organic systems subjected to restraints as those
we are acquainted with invariably are.

Such economy might be simply illustrated by considering the case
of a reservoir of water elevated above two hydraulic motors, so
that the elevated mass of water possessed gravitational
potential. The available energy here represents the stored-up
energy in the organism. How best may the water be conveyed to the
two motors [the organic systems reacting towards conditions A and
B] so

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that as little energy as possible is lost in transit? If the
motors are near together it is most economical to use the one
conduit, which will distribute the requisite supply of water to
both. If the motors are located far asunder it will be most
economical to lay separate conduits. There is greatest economy in
meeting a plurality of functions by the same train of
physiological processes where this is consistent with meeting
other demands necessitated by external or internal conditions.

But an important and obvious consequence arises in the supply of
the two motors from the one conduit. We cannot work one motor
without working the other. If we open a valve in the conduit both
motors start into motion and begin consuming the energy stored in
the tank. And although they may both under one set of conditions
be doing useful and necessary work, in some other set of
conditions it may be needless for both to be driven.

This last fact is an illustration of a consideration which must
enter into the phenomenon which an eminent biologist speaks of as
physiological or unconscious "memory,"[1] For the development of
the organism from the ovum is but the starting of a train of
interdependent events of a complexity depending upon the
experience of the past.

[1] Ewald Hering, quoted by Ray Lankaster, _The Advancement of
Science_, p. 283.

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In short, we may suppose the entire development of the plant,
towards meeting certain groups of external conditions,
physiologically knit together according as Nature tends to
associate certain groups of conditions. Thus, in the case in
point, climatic rigour and scarcity of pollinating agency will
ever be associated; and in the long experience of the past the
most economical physiological attitude towards both is, we may
suppose, adopted; so that the presence of one condition excites
the apparent unconscious memory of the other. In reality the
process of meeting the one condition involves the process and
development for meeting the other.

And this consideration may be extended very generally to such
organisms as can survive under the same associated natural
conditions, for the history of evolution is so long, and the
power of locomotion so essential to the organism at some period
in its life history, that we cannot philosophically assume a
local history for members of a species even if widely severed
geographically at the present day. At some period in the past
then, it is very possible that the individuals today thriving at
Paris, acquired the experience called out at Upsala. The
perfection of physiological memory inspires no limit to the date
at which this may have occurred—possibly the result of a
succession of severe seasons at Paris; possibly the result of
migrations —and the seed of many flowering plants possess means
of migration only inferior to those possessed by the flying and
swimming animals. But, again, possibly the experi-