148 
BIOLOGY: A. J. LOTKA 
Proc. N. a. S. 
about 1.37 X 10^^ kilogramcalories per annum, enough to support a popula- 
tion of about 105 million persons (equivalent to about 88 million adults) 
at the present rate of food consumption (4,270 kilogram-calories per adult 
per day). Suppose, as a simple, though rather extreme illustration, 
that man found means of doubling the rate of growth of crops, and of 
growing two crops a year instead of one. Then, without changing the 
average crop actually standing on the fields, the land would be capable 
of supporting double the present population. If this population were 
attained, the energy flux through the system composed of the human 
population and the organisms upon which it is dependent for food, would 
also be doubled. This result would be attained, not by doubling the mass 
of the system (for the matter locked up in crops, etc., at a given moment 
would be, on an average, unchanged) but by increasing the velocity of 
circulation of mass through the life cycle in the system. Once more it 
is evident that, whenever a group of^ organisms arises which is so consti- 
tuted as to increase the rate of circulation of matter through the system 
in the manner exemplified, natural selection will operate to preserve and 
increase such a group, provided always that there is presented a residue 
of untapped available energy, and, where circumstances require it, also a 
residue of mass suitable for the composition of living matter. 
To recapitulate: In every instance considered, natural selection will 
so operate as to increase the total mass of the organic system, to increase 
the rate of circulation of matter through the system, and to increase the 
total energy flux through the system, so long as there is presented an un- 
utilized residue of matter and available energy. 
This may be expressed by saying that natural selection tends to make the 
energy flux through the system a maximum, so far as compatible with the 
constraints to which the system is subject. 
It is not lawful to infer immediately that evolution tends thus to make 
this energy flux a maximum. For in evolution two kinds of influences are 
at work: selecting influences, and generating influences. The former 
select, the latter furnish the material for selection. 
If the material furnished for selection is strictly limited, as in the case 
of a simple chemical reaction,^ which gives rise to a finite number of prod- 
ucts, the range of operation of the selective influences is equally limited. 
In the case of organic evolution the situation is very different. We 
have no reason to suppose that there is any finite limit to the number of 
possible types of organisms. In the present state of our knowledge, or 
rather our ignorance, regarding the generating influences that furnish 
material for natural selection, for organic evolution, an element of uncer- 
tainty enters here. It appears, however, at least a priori probable that, 
among the certainly very large (if not infinite) variety of types presented 
for selection, sooner or later those will occur which give the opportunity for 
