926 The American Naturalıst. [October, 
It seems to the writer that Mr. Dall has not given the full value to 
pathologic cases. These may result from unfavorable surroundings ; 
or at the close of the natural life of a species or group, pathologic 
changes may be taken on, as clearly shown by Prof. Hyatt in fossil Ce- 
phalopods. In these cases, all the individuals of a race and successive 
generations in a given locality or geological horizon may be induced 
to take on features of a pathologic character, and form a — 
series of individuals, species, or genera. 
In studies of the development of the hinge of Pelecypods, and the 
columellar plaits of Voluta, Mitra, and other Gastropods, Mr. Dall gives 
concrete examples of dynamical genesis. These appeal strongly iif 
in favor of this hypothesis, on account of the simplicity of the prob- 
lem as he presents it, and the complete way in which the facts illustrate 
the mechanical stresses to which the parts have been exposed. 
Mr. Dall’s paper suggests to me what I believe is a new way in 
which to consider natural selection in its relation to dynamic influ- 
ences, In their mutual relation it is clear that natural selection is not 
a new force coming in where dynamic influences cease ; but rather it 
comes as a corollary of dynamic influences. It is not a foreign force, 
acting in some other and special way ; but it acts in harmony with, 
and as a natural outcome of dynamic influences, If dynamic forces 
tend to push a series of organisms in any given time of variation, some 
individuals will evidently be pushed further on that line of variation 
than other individuals, on account of greater plasticity or other-causes. 
Those which have yielded most fully to the acting forces will be as a ne- 
cessity more completely in harmony with the mechanical requirements 
of the environment ; therefore they will be more likely to successfully 
propagate and hand down the modified features which fit them for the 
environment, and have been fitted on to them by the environment. 
That such individuals will propagate especially freely may be legiti- 
mately inferred from the well-known principle, that complete har- 
mony with environment is one of the prime factors in the successful 
reproduction of animals and plants. Such especially well-fitted indi- 
viduals are not exceptional and sporadic cases; they are in direct 
accord with the ascendant line of the series to which they belong, and 
would therefore be naturally selected because they have most com- 
pletely filled the mechanical requirements of their environment. 
During successive generations individual advantages naturally selected 
on this principle will not tend to become merged and lost sight of in 
the general average; they will tend to elevate and bring into greater 
of equilibrium the oncoming generations. 
ROBERT T, JACKSON. 
