THE PRINCIPLES OF BIOLOGY. 
213 
into play certain counter forces which eventually out-balance 
it and initiate a movement in the opposite direction. The 
dangers* which a species has to combat, and its ability to 
combat them, must each on the average remain at a constant 
level. 
If the reproduction of a Hydra be imagined to proceed 
as slowly as that of Man, the race would immediately die 
out. On the other hand, did Oxen propagate as fast as 
Infusoria, thev would all die of starvation in a week. As the 
ability of a species to meet the forces which tend to destruc¬ 
tion is a constant quantity, and this comprises two factors— 
power to maintain individual life, and power to multiply—- 
these must vary inversely as one another; one must increase 
as the other decreases. In other words, as high organisation 
implies great capacity for self-preservation, a correspondingly 
low degree of fertility will suffice to maintain the numbers; 
and in a lowly organised creature, whose power to contend 
against the adverse forces of the environment is small, great 
fertility must exist to compensate for the increased mortality. 
But not only is this so : it is possible to see why it is so. 
Each organism having, on the average, a iixed power of 
obtaining nutriment—that is, of acquiring energy; the more 
it uses up this stock of energy for its own processes of life, 
the less will it have to spare for producing new individuals. 
For, as has been shown in previous chapters, Genesis, by 
whatever mode it may be carried on, is a process of disinte¬ 
gration, and is thereby essentially opposed to that integration 
which is one of the elements of Evolution. The other 
element, differentiation—or the division of labour between 
the parts of an organism—implies a transfer and transforma¬ 
tion of food which also requires the expenditure of energy. 
Both elements tend to prolong the individual life, but only at 
the cost of the store of energy which can be reserved for 
Genesis. Individuation, therefore, and Genesis must neces¬ 
sarily vary in an inverse ratio— i.e., I oc ~ or IG= constant. 
This relation, however, though nearly true, is not exactly 
so. Each advance in Evolution implies economy, and thus 
the product of I and G, instead of remaining exactly constant, 
increases slightly with each advance in organisation, and this 
surplus becomes an admirable self-acting tendency to further 
the supremacy of the most developed types. 
* Under this head natural death is included, for this only differs 
from what is called accidental death in the smallness of the cause 
which proximately induces the effect. I may note that I have, as far 
as possible, used Mr. Spencer’s own words in stating his opinions. 
