34 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
the rapid changes of environment faunas have shifted their habita- 
tion, the conclusion would be that there was a slight acceleration in 
evolution with the readjustment of the faunas, and that the shiftings, 
when of a general nature, would result in modifications of the faunas 
which would serve as means of a closer correlation of the time rela- 
tions of geological events, not only in one quarter, but quite around 
the globe. AVhile the first of these inferences is not inconsistent with 
the second, the first does not furnish an explanation of the constant 
considerable change of genera as well as species seen on comparing 
the successive faunas of any continuous section if followed through 
several hundred feet of diverse sediments. In either case the observ- 
ing and the recording of the differences expressed by fossil faunas of 
the same horizon coincident with geographical distribution promise 
to throw some light on the problems of time measurement of organic 
evolution and to test the value of fossils as means of geological 
correlation. 
The possibility that a fauna may preserve its integrity by shifting 
its habitation with the slow changes of environmental conditions was 
suggested by Barrande's theory of colonies. He believed that a fauna 
characteristic of one epoch of time, by isolation, could be preserved 
in a restricted basin, while all the general faunas were destroyed and 
replaced by others, and that later, in a second or third epoch, the 
representatives of the preserved "colony" might migrate into the 
general seas and reappear (out of stratigraphical place) in the midst 
of the succeeding faunas. The theory as a whole did not commend 
itself to general acceptance. But " recurrence of fossils," the fact at 
the basis of his theory, has been frequently recorded; and the theory 
that a fauna may be preserved in one region later than in another 
appears to have much evidence to support it. Barrande was, how- 
ever, not an evolutionist; uniformity and continuity of species was 
a part of his creed; hence he did not consider the positive aspect of 
the case, nor did he conceive change of environment to be a cause of 
modification; he saw only the negative side, viz, the association 
of uniformity of conditions with preservation of characters among 
the inhabitants. This conception of the unchanging character of the 
species still continues to influence general notions of correlation, 
although we are theoretically all evolutionists. 
Correlation by identity of species implies that the rocks contain- 
ing the same species of fossils were formed at the same period of time, 
and on this basis it is inferred that formations belong to the same 
geological horizon so long as their species are found to be the same. 
While in a general way this is correct, since the evolution of forms 
goes on at a very slow rate, the converse is not true, viz, that unlike- 
ness of species is evidence of a different age for the formations hold- 
ing them. Sufficient facts are now gathered to prove that in each 
great province different faunas, adjusted to the different conditions of 
