18 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
tional history, it becomes necessaiy to notice the fact that each indi- 
vidual organism expresses the characters by which the taxonomic 
divisions of all ranks are defined. When one speaks of a species 
living in a certain locality or at a particular period of time, the 
expression is not strictly true; the species (or the genus) is a cate- 
gory, not a living body. 
The fact in the case is that individuals live, developing the charac- 
ters of some species, or of the specific category. Each individual is no 
more. a species than it is a genus or order or class; and whenever one 
is speaking of the time range of a genus or species, it is necessary to 
understand that what is meant is the time range of the particular 
specific or generic characters, as the case may be. By forgetting this 
point one is liable to think that the species cited as characteristic of 
a particular epoch of geological time suddenly became extinct when 
the formation holding it is succeeded by another containing different 
species. 
So long as representatives of a genus continue to appear it is 
necessary to assume that there has been a continuous succession of 
living individuals arising by direct generation one from another. 
Whenever a new species appears in the rocks it is not to be supposed 
that it had no immediate ancestors living at the time of sedimenta- 
tion of the subjacent formations. So long as a family exists in the 
world, it is also necessary to assume that genera and species have 
continuously existed, and their absence from the formations does not 
indicate that they did not live in the zones of sedimentation which 
lack their remains. 
These observations make plain the reason for the introduction of 
the ideas expressed by the terms migration and shifting of faunas, to 
account for absence of faunas, in the place of the idea of extinction 
held by the earlier geologists. Not onty must we conceive of whole 
faunas, as well as individual species, migrating, but it is necessary 
to assume that, coexistent with thick formations that are barren of 
fossils there were living, in probably not very distant localities, faunas 
made up of abundant individuals of many kinds of different species 
and genera. This fact will explain also why it is necessary to take 
into consideration the question of migration in order to make corre- 
lations with precision. Other problems, which will be discussed 
farther on, are suggested by the fact that the evolutional accounting 
for divergence of characters implies always a continuous, unbroken 
series of generations for each race of organisms until it becomes 
extinct. The characters which are of specific rank at one time in the 
history of a race can not take generic rank in another part of the his- 
tory. The passage from varietal to specific rank, advocated by Darwin 
in the "Origin" as the mode by which species originate, does not apply 
to specific characters, since the reason for the distinction between 
variety and species is, so far as the characters are concerned, purely a 
