228 ANIMALS OF THE PAST 
Then, too, while a tendency to vary seems to 
be inherent in animals, some appear to be vastly 
more susceptible than others to outside influ- 
- ences, to respond much more readily to any 
change in the world about them. In fact, Pro- 
fessor Cook has recently suggested that the in- 
born tendency to variation is sufficient in itself 
to account for evolution, this tendency being 
either repressed or stimulated as external con- 
ditions are stable or variable. 
The more uniform the surrounding condi- 
tions, and the simpler the animal, the smaller — 
is the liability to change, and some animals 
that dwell in the depths of the ocean, where 
light and temperature vary little, if any, re- 
main at a standstill for long periods of time. 
The genus Lingula, a small shell, traces its 
ancestry back nearly to the base of the Ordo- 
vician system of rocks, an almost inconceivable 
lapse of time, while one species of brachiopod 
shell endures unchanged from the Trenton 
Limestone to the Lower Carboniferous. In 
the first case one species has been replaced by 
another, so that the shell of to-day is not ex 
actly like its very remote ancestor, but that 
