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 

 T^imestone to the I^ower Carboniferous. In 

 the flrst case one species has been replaced by 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 



