828 THE STRUCTURE OF EVOLUTIONARY THEORY
be construed as a genuine phenomenon, actively maintained—and not as an
absence of anything. Cronin writes (1985, pp. 60-61): "Total within-sample
variability representing 10^2 to 10^3 years is only slightly less than variability over
105 to 2 X 10^5 years. Puriana mesacostalis shows no secular trends in its
morphology over this time interval that might be evident from a lack of
concentricity of the ovals—stasis is directionless. Yet high-amplitude
environmental fluctuations occurred during this time that could have catalyzed
speciation or caused extinction."
Once we construe stasis as an interesting evolutionary phenomenon, actively
promoted within species, we then become eager to know more about its fine-scale
anatomy and potential causes. A remarkable series of studies by Michael A. Bell
on the Miocene stickleback fish Gasterosteus doryssus (Bell and Haglund, 1982;
Bell, Baumgartner and Olson, 1985; Bell and Legendre, 1987) provide evidence at
a maximal level of paleontological resolution, for these fossils occur in abundance
in varved sediments with yearly bands— surely a summum bonum for attainable
temporal precision! Bell and colleagues
9 - 14. An impressive demonstration of stasis in the peristome size of the echinoid Discoides
subucula. From Smith and Paul (1985). All specimens are shown for each narrow collecting
interval, spaced one meter apart.