WHY DO ANIMALS BECOME EXTINCT? 229 
the type of shell should have remained un- 
changed when so many other animals have 
arisen, flourished for a time, and_ perished, 
means that there was slight tendency to varia- 
tion, and that the surrounding 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 exter- 
mination might be adduced, but among them 
that of the tile-fish is perhaps the most strik- 
ing. ‘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 considerable 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 brought 
