Peirce. — Studies of Irritability in Plants . 463 
is attributed. Or others, attempting to reduce the process to greater 
precision, would designate in terms of still smaller units — namely the 
chemical compounds concerned — the means of continuance of bodily 
characters. But since a chemical reaction depends upon the substances 
employed and also upon the conditions prevailing at the time, it is only 
where like substances act upon each other under like conditions that the 
results are alike. It is therefore as important to study the conditions of 
life as its physical basis in order to understand the phenomena of life. 
Analytic and experimental study of the environment will enable us, in the 
course of time, to reduce to definite terms of physics, as well as of 
chemistry, much of what is now indefinitely summed up under the name 
of heredity. We shall see that, along with continuity of substance, the 
continuity of influence must be reckoned as indispensable to heredity ; 
that, indeed, the continuity of influence is part, and a large part, of 
heredity. 
I have shown in the foregoing pages that, according as the direction 
of illumination is usual or unusual, certain plants have their normal form or 
some other wholly different. Whether we regard illumination from only 
one direction as merely a condition or as a direct stimulus to the develop- 
ment of dorsiventrality in Anthoceros , it is evident that, unless the young 
plants developing from the spore are exposed to influences like those under 
which their parents developed, they will be unlike their parents. So far as 
two species of plants are concerned I have furnished experimental evidence 
in favour of the hypothesis previously advanced 1 that, in addition to what 
is actually transmitted from parents to offspring in the continuity of sub- 
stance, the likeness of parents and offspring is due to the likeness (or identity) 
of influence to which succeeding generations are exposed. In other words, 
certain physical factors of the environment, constant or periodic but unchang- 
ing, constitute means of repeating parental characters generation after 
generation, and these environmental influences are as essential as the substance. 
Given the same chemical compounds and the same arrangement of these in 
the fertilized egg as in the parents, the young must be like the parents 
if their environment is the same. But if any factor vary — in the chemical 
composition or in the structure of the fertilized egg, or in the conditions 
under which the young develop — the young will be proportionally different 
from the parents. The individual young will differ not only from their 
parents but also from each other. We are accustomed to these differences 
among the individuals of a brood. We are in the habit of thinking of the 
environment as introducing variety. The changing factors of the environ- 
ment do introduce variety. So do the changing compounds or proportions 
of compounds in the bodies of the parents and in the fertilized eggs which 
1 Farmer, J. B., On stimulus and mechanism as factors in organization, The New Phytologist, 
ii, Nov. and Dec., 1903. Peirce, G. J., Text-book of Plant Physiology, pp. 279-83, May, 1903; 
Certain undetermined factors in heredity and environment, Amer. Naturalist, xxxviii, April 1904. 
