180 ANIMALS OF THE PAST 



either repressed or stimi:Iated as external conditions 

 are stable or variable. 



The more uniform the surrounding conditions, and 

 the simpler the animal, the smaller is the liability to 

 change, and some animals that dwell in the depths of 

 the ocean, where Hght and temperature vary little, if 

 any, remain at a standstill for long periods of time. 



The genus Lingula, a small shell, traces its ancestry 

 back nearly to the base of the Ordovician 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 the first case one species has been replaced by another, 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 exactly hke its very re- 

 mote ancestor, but that the type of shell should have 

 remained unchanged when so many other animals have 

 arisen, flourished for a time, and perished, means that 

 there was slight tendency to variation, and that the sur- 

 rounding conditions were uniform. Says Professor 

 Brooks, speaking of Lingula: "The everlasting hills 

 are the type of venerable antiquity; but Lingula has 

 seen the continents grow up, and has maintained its in- 

 tegrity unmoved by the convulsions which have given 

 the crust of the earth its present form." 



Many instances of sudden but local extermination 

 might be adduced, but among them that of the tile-fish 

 is perhaps the most striking. This fish, belonging to a 

 tropical family having its headquarters in the Gulf of 

 Mexico, was discovered in 1879 in moderately deep 

 water to the southward of Massachusetts and on the 

 edge of the Gulf Stream, where it was taken in consider- 

 able numbers. In the spring of 1882 vessels arriving at 

 New York reported having passed through great 

 numbers of dead and dying fishes, the water being 

 thickly dotted with them for miles. From samples 



