150 Scientific Proceedings, Royal Dublin Society. 
morphology and physiology are favourable, the ideally economical 
system will be attained when in place of two separate processes, 
a, (3, the one process y, cheaper than a+ (3, 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,’’ and although expressed here rather crudely, it is to 
be expected with certainty (I venture to think) 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 daily gains or the stored-up reserve material of the organism]. 
How best may the water be conveyed to the two motors [the or- 
ganic reactions towards conditions A and B] so that as little head 
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 two pipes. [There 
is greatest economy in meeting a plurality of functions by the 
same train of physiological processes where this is consistent with 
discharging other functions necessitated by external or internal 
conditions. | _ A 
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. Ii we open a valve inthe conduit 
both motors start into motion and begin consuming the energy stored 
in the tank. And although they may both under one set of con- 
ditions be doing useful and necessary work for us, in some other 
set of conditions it may be quite 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,” and illustrates that in 
the organism its development from the ovum is but the starting of 
a train of interdependent events of a complexity depending upon 
the experience of the past. 
1 Professor Hering, quoted by Professor Ray Lankaster, ‘‘ The Advancement of 
Science,’’ p. 283. 
