580 THE STRUCTURE OF EVOLUTIONARY THEORY
p. 28) writes, "all this exploring, stopping, and rushing, in the pursuit of profitable
adaptation, has resulted in the great family tree of the animals."
Nelson et al. (1967, p. 239) briefly extol the full sequence—from the rule of
selection in local populations, through speciation, to the origin and diversification
of phyla: "Evolution in its simplest and broadest sense means changes in gene
frequency over a period of time. Natural selection guides these changes... Over
long periods the accumulation of changes may be sufficient to separate once
similar populations into distinct groups. In the course of evolutionary history this
divergence has apparently led to different classes (mammals, birds, fish, etc.),
different phyla (insects and corals, for example), and even different kingdoms
(plants and animals)."
The dominant high school text of the 1960's and 70's depicts the standard
equine example of macroevolution as anagenetic gradualism guided by natural
selection, thus making any definition of chronospecies arbitrary: "The fossil record
shows that all these differences are the result of a series of many gradual changes.
Each change that became established through natural selection must have been
very slight; only when many such changes accumulated did they result in
detectable differences. How can this long sequence of horses be divided into
species?" (Biological Sciences Curriculum Study, Green Version, 1973, p. 621).
The accompanying figure of the phylogeny of horses depicts the actual (and
copiously branching) bush as a smooth ladder of progress (see Fig. 7-3).
Bonner (1962, pp. 52-53), another leading evolutionist who also wrote a
popular text, argued that paleontologists can't study the mechanics of evolution
directly, but professed complete confidence in the efficacy of microevolutionary
selection:
Paleontologists as well as ecologists have been for some years studying the
evolutionary factors we have discussed, and have continuously attempted to
see how the fossil record, on the one hand, or the present-day distribution
of animals and plants, on the other, fit in with this scheme. There seem to
be no major discrepancies, and a general feeling that the mechanism of
evolution is understood prevails, particularly in regard to the importance of
selection and the method of formation of new species... Some groups such
as the mollusks have been exceedingly slow in their progress while others,
such as the mammals, have been very rapid. Again this can be totally
understood in terms of selection in particular environments. No other
hypothetical mechanisms seem to be necessary to account for the facts, as
we know them.Such confidence in microevolutionary sufficiency can only lead to a down-
grading of paleontology—either to theoretical irrelevance, or to a status as a mere
repository for results of processes that can only be elucidated by studying modern
organisms (and may then be smoothly extrapolated across a million millennia). I
do not think that this derogatory judgment originated by the conscious intent of
most textbook authors. Rather, the marginalization of paleontology flows directly
from the logic of pure extrapolation. The basic