TIME IN EVOLUTION—ZEUNER 255 
COELACANTHIDAE 
SRT RES TE 
250 200 150 100 50 1@} 
Diplocercididae 
_ 300 
million years 
DEV, CARBON. PERM. TRIAS. JURASS.CRET. CAINOZOIC 
Ficure 4.—Diagram showing frequency of species of coelacanth fishes in relation 
to time. Inthe Upper Devonian the family Diplocercididae flourished. In the 
Carboniferous it was replaced by the Coelacanthidae, which reached their climax 
in the Triassic. The small number of coelacanths recorded from the Permian 
may not represent their true frequency owing to the fact that relatively few 
marine deposits are known from the Permian. Possibly, therefore, the rise to 
the climax in the Triassic was not interrupted in the Permian, as is shown in 
the diagram, but was continuous from the Lower Carboniferous; if so, the rise 
would have taken about 100 million years and the rate of evolution would then 
be one of the slowest known. The coelacanths are regarded as a very con- 
servative group which changed but little in the course of time. 
60 50 40 30 86.20 lO oO 
million years 
PALAEOGENE NEOGENE. 
Figure 5.—Diagram showing the number of new species of the molluscan genus 
Poiretia appearing in the different subdivisions of the Tertiary. The maximum 
number of new species was produced in the late Palaegene, within 30 to 40 
million years of the appearance of the genus. 
