Grades of Evolution 
THE OZARKIAN AND ITS SIGNIFICANCE 539 
lines) becomes greater and greater until the end of the cycle, 
when there comes a critical period of wide migrations of species 
(represented by the horizontal arrows), a mingling of faunas, a 
fiercer struggle for life, together with the more active operation 
of other factors of evolution, and a consequent hastening of the 
steps of evolution everywhere, but especially in the lagging 
areas, to a more or less even general line. In the second cycle 
N- Am * Europe S Afr? Austr 
Fic. 3. Diagram showing the general relations of synchrony to homotaxy in 
geological times. Full lines represent the same time in different places (synchrony), 
Horizontal dotted lines represent equal stages in evolution (homotaxy). 
the same process begins and progresses, except that now America 
is ahead, and increasingly so until the next crisis; when the syn- 
chronic lines are again evened up to parallelism with the homo- 
taxic. If it were not for these occasional evening-ups a general 
geological history based on organic remains would be impossible. 
The whole process may be likened to a long army line 
marching abreast over a broken country, led by officers, who 
may be compared to the dominant types. Soon the line 
Graces. or S2Volui1 om 
