iS9J.] 85 [Hyatt. 
of sucli repeated actions, developing into hal)its, and that these 
necessarily end in the permanent establishment or fixing of these 
modifications in varieties and species. 
This theory accounts satisfactorily for the so-called mys- 
terious suitability of organic structures for the work they have to 
do. Such a force, capable of producing changes of structure and 
sensitive to the impinging action of external physical conditions, 
must work in directions determined by these two factors, i. e., tlie 
structures already existent in the organism and the external forces 
themselves. Tt is obvious that these actions and reactions must 
])roduce habits and changes of structure which are direct 
responses to the environment. To use the Darwinian j)hraseol- 
ogy, one can say that the variations tiius produced are one class 
of natural selections, and T have called them in other publications 
phi/sical selections, although it is likely that the use of the word 
selection in any way may convey an erroneous idea of my mean- 
ing. Selection implies the choice of some characters or tendencies 
out of a number of others, and in the minds of most naturalists it 
also implies the survival of the fittest chosen by the Avorking of 
the struggle for existence in two directions, in one direction, 
Itetween contending organisms, and in the other, between the 
same organisms and their surroundings. 
The above was written before I read ''Energy as a factor 
in organic evolution,"^ in which Dr. John A. Ryder discusses 
the relations of the statical and dynamical phenomena of devel- 
opment and evolution, using the terms ergogeny and ergo- 
genetic for all the modifications produced by organic energy, and 
considers kinetogenesis and statogenesis as divisions of the first 
named. I'hese uistructive speculations and observations were 
written to show that the changes of form produced hy motion, and 
those modifications or conditions which may be properly con- 
sidered as due to the conditions of equilibrium, are often i-eached, 
as is claimed by Ryder, as the result of kinetogenesis and are 
considered by him as statogenetic. These are interesting in 
connection Avitli the above, and sup[>ort the remarks made 
with reference to the use of terms like "avolution," and are 
substantially in agreement with the general views taken in 
this essay, although taking up a side of the mechanics of 
evolution not discussed hei'e. 
1 Proc. Amer. philos. soc. Philadelphia, v. ol ; reprint May 18, 1893. 
