WHY DO ANlxMALS 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." 



JNIany 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 inoderately deep 

 water to the southward of JNIassachusetts 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 



