538 JOSE PETER GON TLE 
evolution.— We have already explained in a previous paper’ how, 
during the course of a cycle, the geographical diversity of 
organic forms in isolated regions becomes greater and greater 
indefinitely as long as isolation continues, until finally synchronic 
correlation of strata in different regions becomes difficult or 
impossible. But that during critical periods (and to less degree 
at other times) there occur wide migrations and mingling of 
faunas and corresponding obliteration of geographical diversity, 
only to commence again with new isolations and a new geo- 
graphical diversity increasing again with time. ‘Thus there are 
alternations of increase and obliteration of faunal diversity. This 
idea has an important bearing on the doctrines of synchrony and 
homotaxy. At the beginning of a great cycle immediately after 
a critical period, geographical faunas commence, as it were, all 
abreast; synchrony and homotaxy are now in harmony. As 
time goes on, the newly mingled but re-isolated faunas develop 
in different directions and at different rates, become more and 
more divergent in character, and more and more different in 
grade of evolution. Synchrony and homotaxy become more 
and more discordant, until at the end of the cycle it becomes 
extremely difficult or even impossible to correlate strata of dif- 
ferent countries synchronically. Then there comes another 
critical period of widespread oscillations of crust and readjust- 
ment to new conditions of equilibrium with accompanying oscil- 
lations of temperature and wide migrations of species and 
mingling of faunas and floras, hastening the steps of evolution 
everywhere, but obliterating geographical diversity, and, as it 
were, evening up again synchrony and homotaxy, only to com- 
mence a new cycle by re-isolation. 
An attempt is made to roughly represent this process by a 
diagram (Fig. 3). In this diagram two cycles are represented. 
In the first, Europe is ahead, and increasingly so as the cycle 
goes on, and Australia is most lagging. In the second, North 
America is ahead. In each the divergence between synchrony 
(the full waving lines) and homotaxy (the horizontal dotted 
* Bull. Geol. Dept. of Univ. of Cal., Vol. I, p. 314, 1895. 
