Jan., 1922] PULLING — BIOPHYSICS IN PLANT PHYSIOLOGY 
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system of the class is capable of manifesting the activity that identifies the 
class. She does this without anticipating any theory to account for the 
activity and without considering that, with respect to other activities, 
these individual systems of the class may be wholly unrelated. 
It is sometimes doubted that this is done in physics. Physics is reputed 
to be in constant search for explanations of phenomena. That, of course, 
is true, and no science can abandon attempts along this line. But it is 
likewise true that physics does not make the search for explanation as the 
first step. The first step is the formulation of a quantitative law that 
represents the relation between the activity characteristic of the class and 
the quantity that symbolically represents the individual system, in other 
words, a constant whose magnitude may vary in any way whatever from 
system to system of the same class. When this law is established, the 
task is undertaken of discovering the law that connects the magnitude of 
the constant with some measurable feature of the individual system, some 
characteristic that can be measured when the system is not manifesting 
the activity under consideration or which for some other reason renders 
calculation of the constant easier. 
Somewhere, at some time during this undertaking, an observation is 
made that leads an experimenter to connect the behavior of systems in 
one class with that of systems in another. Gradually thus relationships 
are perceived, the statements of which are, in fact, the so-called explana- 
tions, each law being seen to be a special case of another and "explained" 
by the more general. I do not mean that each individual physicist pro- 
gresses in this way, but that, on the whole, this faith that careful measure- 
ment of the interrelations of activities of individual systems, the expression 
of the quantitative relations between the attributes of these systems, 
always with the view of defining the particular systems each by the magni- 
tude of a measurable dimension, a static character, etc., or by a relation 
between such characters, a constant in short — faith in this method has 
been the keystone of progress in physics. 
Because this may appear to be but the opinion of an outsider, perhaps 
it would be best to quote a physicist. Clerk Maxwell's classification of 
the physical sciences may be summarized thus : 
The chief divisions of physics are two : 
A. Fundamental science of dynamics, or the doctrine of the motions 
of bodies as affected by force. 
B. Secondary physical sciences. Each has two divisions or stages. 
To quote : ^ 
In the elementary stage it is occupied in deducing from the observed phenomena certain 
general laws and then employing these laws in the calculation of all varieties of phenomena- 
In the dynamical stage the general laws already discovered are analyzed and shown to be 
equivalent to certain forms of the dynamical relations of a connected system and the attempt 
2 Maxwell, J. Clerk. Physical sciences. Encyc. Brit. 19: 1-3. The R. S. Peale reprint 
of the 9th edition. Chicago, 1892. 
