128 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
the term race. So it is particularly the single generation rather than 
the single individual that we have in mind when the time value of 
an individual is under consideration. 
If we could actually know the number of generations it takes to 
accomplish changes sufficient to be marked by describing the two 
extreme individuals as of different species, then we could express by 
such a number the magnitude of difference between the time values 
of the individual and of the species. The best we can do is to state 
that the two measures are of a different order of value. We may state 
that the time length during which the average species reproduces 
its kind without appreciable deviation in its specific character is 
measured by thousands and possibly millions of generations, while a 
single generation measures the time length of the first order in little- 
ness of value associated with the individual. If we could deal 
with it in geology, the life period of the individual would be the 
primary unit of the bionic system (monobiochron). But as this can not 
be ascertained by the study of fossils— dead remains of organisms — 
we must take for the lowest practical bionic unit some unit which 
is capable of expression by fossils (dibiochron). This shortest lapse 
of time, to which the fossils themselves may give expression, is 
associated with the continuous life of the species, and may be con- 
ceived of as directly determined by the relative vigor maintained by 
the individuals struggling with one another at the particular point of 
time recorded. So long as, at a particular spot (a), under what may 
be supposed to be unchanged, local, environmental conditions (b), the 
relative number of individuals of each species (c), with the same corn- 
parat ive size and proportions of form (d), continues unchanged, so long 
a certain small unit of time may be considered to have elapsed. This 
is called a dibiochron, because it is the measure of the second order 
of appreciable magnitude of the expression of the bionic, or endur- 
ance qualities of the organisms whose fossil remains are examined. 
The definition of terms was given in a previous paper", an extract 
of which will explain the sense in which the terms are used: 
In order to isolate this time quality I have proposed to speak of it as the bionic 
quality or value of the organism. The bionic quality of an organism may, then, 
be defined as its quality of continuing, and repeating in successive generations, 
the same morphologic characters. * * * And if we should adopt the name 
chron to apply to geological time-units in general, and biochron to the units whose 
measure is the endurance of organic characters, we have a means of constructing 
a system of nomenclature which will express what is now known of geological 
time relations, and (more important still), which will serve as an aid in accumu- 
lating the necessary statistics to perfect the geological time-scale. 
Order of magnitude of bionic units. — In expanding this system of nomenclature 
the following table will indicate the principle upon which the fundamental units 
of time value will be discriminated and named. The time-unit of lowest rank 
will he based upon the life endurance of an individual organism; the amount of 
« Jour. Geol., Vol. IX, p. 579. 
