[278]

Nov. 55

- say there are 50,000 species –

Plants have an average say 8 species per genus – at period when the 50,000 have become extinct & changed into new forms; the genera will have still 8 species, but then will be many new genera – Then new genera [being] descent from some one old species, say half genera will be new, then implied death of 25,000 species. – (Extinction will fall on small genera with few species, which will make gaps more complete.) allied species will give rise to new genera, so that suppose one genera gave rise to 3 or 4 new ones with an average 24 or 32 species – This will imply destruction of 24 or 32 species ∴ if 2 or 3 genera, or even more in smaller genera will be most destroyed & therefore.

[278v]

great gaps rapidly made.