both in the sense that a tachytelic line
gradually becomes horotelic and in the sense that the differences in
degree of adaptive change involved, in rate of evolution reached, and
in clarity of threshold are all on continuous scales. There is no sharp
point where one can say, "Below this is horotely and above it is
tachytely." Nevertheless there is a difference that may be very
appreciable and significant in less marginal cases.
Well-recorded but marginal instances occur in such
sequences as that of the Equidae (Chapter VIII). The changeover from
browsing to grazing has a threshold effect and involves a definite,
temporary increase in evolutionary rate. This verges, at least, on
tachytely. The even more rapid changes from one stable type of foot
mechanism to another are still more clear-cut examples of this sort of
phenomenon and might well be designated as rather small-scale tachytely.
Another well-recorded example of small-scale tachytely, leading to
extinction in this case as tachytely often does, is that of
Gryphaea (Chapter IX). One of the most remarkable known examples
is that of the snail Valenciennesia, formerly classified in
another suborder than the notably different Limnaea, but shown
by Gorjanovic-Kramberger (190l, 1923) to have evolved from the latter
so rapidly that the whole process occurred while a horse, Hipparion
gracile, on adjacent lands showed no appreciable change. Here,
too, change to a distinctly different adaptive zone is involved: from
clear and fresh to muddy and brackish water.8
As Stebbins (H)50) has suggested, tachytelic lines
are likely to be, although they are not necessarily, narrowly adapted.
They include the forms that are forced into rapid change by changes in
their adaptive zones. Their adaptive evolution tends to be the opposite
of that seen in bradytely, and the factors involved tend to be inverse.
In the figure of speech utilized above, tachytelic lines hit the baffles
and are deflected into sharply divergent corridors.
Tachytely is defined as a phylogenetic phenomenon
and is to be distinguished from rapid speciation or splitting. Rapid
evolution in the sense of high origination rates, such as occur on
invasion of new and open habitats (e.g., in the Galápagos or Hawaiian
Islands, as discussed before, or rapid development of endemics in
African lakes, see Worthington, 1937, 1940) is a phenomenon quite
distinct from tachytely. Nevertheless, when adaptive radiation occurs,
and especially if this
8 Basse (1938) has cast doubt on Gorjanovic-Kramberger's
interpretation, but Basse's opposing evidence is weak.