126 COKRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
Since it takes an appreciable length of time for an organism to 
develop to maturity the structure by which it carries on its living 
processes, and as, secondly, every individual organism develops its 
form elements by passing from a formless stage into a more and more 
complex morphological stage, these two elements, time and individual 
development, offer promise of some satisfaction for the measurement 
of organic values, which may be considered mathematically. 
Organisms are not to be measured by the amount or kind of matter 
of which their bodies are constructed, but by the disposition and use 
they make of the matter within the scope of their activities. It is 
the shape of the lobster's claw, not its chemical constitution, which is 
significant. 
Following out this line of search, we notice that the vigor expressed 
by the coming to birth and the growing to maturity of a single organ- 
ism is repeated when it reproduces itself in a second generation. 
Whatever value be imagined as the value of the life power, force, or 
energy by which a single germ goes on to maturity, the value is 
doubled when another generation follows, and trebled on the third 
generation. Generation becomes thus the measure of a certain funda- 
mental ability of organic bodies, and each individual organism stands 
for the exertion of a unit of such force. A fossil individual is the 
measure of this unit of organic energy as much as a living individual. 
Again, if each case of reproduction of an organic individual were 
an exact repetition of the preceding case, all organisms would be alike. 
We assume that difference in the forms of organisms is to be accounted 
for by a change in the processes by which the mature body is con- 
structed in the course of individual development. 
If the constructive form of the adult individual organism be an 
expression of a unit of vital force, it may be assumed that the diver- 
sion of the process of development, so as to modify the construction 
and form, is the expression of another unit of force of some propor- 
tionate relation to the first unit. 
If organic generation goes on for 100 generations without noticeable 
deviation, this second mode of energy may be supposed to be less than 
if some deviation be noticed in the course of 10 generations. The 
evolutional energy expressed in the deviation from a given form in 
the course of repeated generations is of the same nature as that 
expressed by the development of the germ to adulthood, since it is 
morphologically an acquirement of structure or of difference of form. 
This form is visible and is preserved in the fossil as well as expressed 
in the living organism. Hence it is evident that difference in form, 
when it is combined with numbers of generations taken for producing 
the difference, becomes a means by which the relative values of organ- 
isms may be compared. Difference in form is the basis of classifica- 
tion of organisms in systematic zoology and systematic botany. In 
these sciences relative difference in form is expressed by the terms of 
