924 | The American Naturalist. [October, 
RECENT LITERATURE. 
Dall on Dynamic Influences in Evolution.’ —In this inter- 
esting paper we find that Mr. Dall turns to mechanical influences of 
environment as an essential factor in evolution. 
It is generally admitted, he says, that in natural selection we have 
a theory which accounts for the perpetuation of favorable, and the 
elimination, in the long run, of unfavorable variations in organic 
beings. It is equally admitted that the origin of variation is not 
accounted for by this theory. To round out our conception of the 
mode of evolution it is necessary that this deficiency should be sup- 
plied. It should also be supplemented by some conception of the 
mode by which variation is sustained in a given direction until it has 
reached a point of usefulness sufficiently marked to enable the selective 
process to operate. 
He accepts the relation of the organism to its environment as the 
desired factor, and for this adopt the name of Dynamic Influences. He 
maintains that acquired characteristics are inherited. The organism 
suffers during its entire existence a continuous series of mechanical 
impacts, none the less real because invisible. Since individual organ- 
isms usually appear free to wander or remain quiescent, the idea that 
they are under constant stress does not ordinarily suggest itself, and to 
this fact he ascribes the slowness with which the dynamic element in 
evolution has received recognition. The characters developed in an 
organism in response to impacts are acquired, but that which is trans- 
mitted is a facility of response in the same line. ‘This, under favor- 
able conditions and a series of generations subjected to similar 
impacts, may promote and establish the physiological habit, which is 
the directive influence towards the development of the characters in 
question. 
The dynamics of environment vary within comparatively narrow 
limits, when consistent with organic existence. On the other hand, 
owing to the narrowness of the limits, the dynamic variations to which 
organic forms are subjected become relatively more important. It is 
probable that no two organisms have ever been subjected to exactly 
the same dynamic influences during their development. Differences 
of impact necessarily imply differences of response, hence variation is 
inevitable. The origin of variation, therefore, presents no difficulties. 
1 Read before the Biological Society of Washington, March 8, 1890, by Wm. H. Dall. 
