SELECTION AS AN EXPLANATION. 29 
(3) So far as statistical observation has been directed to this sub- 
ject, I believe it has been found that complete correspondence of 
. averages is found in the measurements of mankind only when the 
groups compared are sections of one homogeneous community thor- 
oughly related by community of descent. There is, therefore, reason 
to believe that the laws of heredity check divergence and secure uni- 
formity in proportion to free intermarriage and community of descent, 
and that, if complete separation exists for many generations between 
two groups of the same race, divergence will take place though the 
external conditions are the same. 
We therefore arrive at the conclusion that, while variation and 
isolation are the essential factors in diversity of evolution, and inter- 
crossing and unity of descent the essential agents in uniformity of 
evolution, natural selection may be an important ally on either side. 
II. SELECTION AS AN EXPLANATION OF EVOLUTION. 
In discussing this subject we shall consider, 1, What selection in 
its different forms does not explain; and 2, How far selection is deter- 
mined by external nature. 
1. What Selection does not Explain. 
(1) It can not account for the introduction of cooperative and antict- 
patory action. 
Evidently it can not account for the powers on which it depends 
for its action. Now, natural selection presupposes the general 
power possessed by every organism, of codperative and anticipatory 
action, based upon a discrimination between the probable results of 
different actions, and directed toward the maintenance of that ideal 
state in the actor which we call life. 
While still in the egg or attached to the parent, anticipating the 
need of organs adapted to a new environment, the organism builds in 
different ways the most wonderful structures, all of which are trans- 
formations of its own simple colorless fluid. This we call growth and 
development. 
Having entered on independent life, it anticipates the tendency of 
work and waste to produce exhaustion, and forefends this result by 
appropriating portions of dead, extraneous fluid matter, transmuting 
it into its own living fluids, from which it rebuilds the wasting 
structures. This is assimilation. 
Anticipating its need of special substances to supply this continual 
consumption, it executes many movements in order to reach advan- 
! 
