A Summary of the facts with numerical details omitted can be given as follows. In the diatoms—

1. Species-lifetimes have been alternatively short or long.

2. The main ratios of shorts to longs, or evanescents to permanents, e/p, are 3:1 for Centrales, 10:6 for Fragilariaceæ, 7:9 for allogamous Pennales. These ratios or nearest approaches to these ratios occur in the above three groups, (a) in each group as a whole; (b) in species-arising in each group per geological subperiod; and (c) in heterogeneous genera in each genus as a whole and in each genus per subperiod. A derivative 1:3 ratio also occurs in some pennate genera. There are some significant and understandable exceptions.