TIME IN EVOLUTION—ZEUNER 253 
Now let us consider a few examples. Figure 2 shows the genus 
Salenia. These sea urchins start in the Lower Cretaceous. The 
number of species rises rapidly to a climax in the Upper Cretaceous 
(“increase phase’”’). Cases like this one, of a rapid, almost sudden, 
blossoming-out of a group were called ‘explosive evolution” by Schin- 
dewolf. We may borrow the term and call the phase of rapid increase 
the explosive phase. In Salenia it is followed by a catastrophic drop 
followed by a slow ‘‘decline phase” leading to almost complete 
extinction. Of the cause of this sudden drop we are completely igno- 
rant; it is an unusual feature, as will be seen when other diagrams are 
considered. 
As regards the increase phase, we learn that in Salenia it lasted for 
about 40 million years. 
Figure 3 shows the Brachiopod genus Lingula. Again it reveals a 
rapid rise in the number of species at the beginning, from the Cam- 
brian to the Ordovician, of the order of 40-50 million years. But 
between the increase and the decline phases, a more or less “‘station- 
ary’’ phase is intercalated, during which the number of species did 
not vary greatly. The decline phase of Lingula set in with the Car- 
boniferous. It is a long-drawn-out phase and continues into the 
present. 
Another interesting case is that of the coelacanths, a group of the 
fringe-finned fishes or Crossopterygii (fig. 4). This group never had 
a truly explosive phase. Nevertheless, there is an increase phase 
from the end of the Devonian to the Triassic, lasting about 100 
million years. 
One more example may be given, the molluscan genus Poiretia 
(fig. 5). Here, the explosive phase in the evolution of the genus 
is 80-40 million years. Other examples confirm that the length of 
the explosive phase is of the order of a few 10-million years, the 
extremes so far found being 30 and 100 million years and the mean 
around 50 million years. 
As an example from the plant kingdom, illustrating at the same 
time a group with stable, geometrical characters living in an environ- 
ment which changes but little, diatoms may be shown. Small in- 
vestigated the chronology of their evolution, and the diagram of 
Hemiaulus shown (fig. 6) is a translation into our method of plotting 
of one of his diagrams. It shows a steepening rise to a climax in the 
Cretaceous. The increase phase lasted about 80 million years, no 
longer than in groups living in less stable environments, but there was 
a pronounced initial lag phase. 
Now it is interesting to consider some higher systematic categories. 
One might expect that the explosive phase in the evolution of higher 
systematic units, like families, orders, classes, is longer than in 
genera. But this is not so. 
