334 A BOOK-LOVER'S HOLIDAYS 



which at intervals sweep off their swarming num- 

 bers. The devastation of these diseases, whether 

 the agents spreading them are insects or still 

 smaller, microscopic creatures, is clearly proved 

 in the case of these North American rabbits 

 and porcupines; probably it explains the tem- 

 porary and local extermination of the Labra- 

 dor meadow-mice after they have risen to the 

 culminating crest of one of those *Svaves of 

 life" described by Doctor Cabot. It has 

 ravaged among big African ruminants on an 

 even more extensive scale than among these 

 North American rodents. Doubtless such dis- 

 ease-devastation has been responsible for the 

 extinction of many, many species in the past; 

 and where for any cause species and individuals 

 became crowded together, or there was an in- 

 crease in moisture and change in temperature, 

 so that the insect carriers of disease became 

 more numerous, the extinction might easily 

 befall more than one species. 



Of course, such epidemic disease is only one 

 of many causes that may produce such exter- 

 mination or reduction in numbers. More effi- 

 cient food rivals may be a factor; just as sheep 

 drive out cattle from the same pasturage, and 

 as, in Australia, rabbits drive out sheep. Or 

 animal foes may be a cause. Fifteen years 



