1. Raup, D.: Biological extinction in earth history. Science, 231, 1528–1533 (1986)
2. The composite comments of Simpson, G.: How many species? Evol. 6, 342 (1952);
3. Cailleux, A.: How many species? Evol. 8, 83–84 (1954);
4. Schopf, T.: A critical assessment of punctuated evolution. I. Duration of taxa. Evol. 36, 1144–1147 (1982) make some such numbers plausible as an order of magnitude estimate. That is, such a number far exceeds the number of currently estimated species.
5. While the argument under consideration has to do with species extinction, the larger problem of evolution — whether gradual or episodic — has to deal very detailedly with the fossil record. There is the latent assumption that the fluctuation in the total number of species available at every point in time is lesser in measure — more slowly varying — than the fluctuations in, say, extinctions. If this is the case, then new niche filling species, e. g., a snall group point evolution, has to follow extinction with relatively short delays, such as delays significantly less than a few My.