I917.J 61 



Examples of the former reason are by no means um-ommoii or difficult 

 of observation ; the black rat (Mhs rati us L.) has been ousted from 

 nmny of its haunts by the brown or Norwegian rat {Mns derumanus) ; 

 the introduction of salmon into New Zealand rivers has caused the 

 destruction, or at any rate the reduction in numl)ers, of native species 

 of fi-esh-water fishes ; the ravages caused by rabbits in Australia have 

 driven the Kangaroo away from many of its old haunts ; and then in 

 countries where new newly-introduced insects have been a plague, e.g., 

 the small Cabbage-white Butterfly (Pieris rapae L.) in Canada, the 

 introduction by man of their appropriate natural parasites, e.g., the 

 Braconid Apanteles glomeratus has speedily caused a reduction in their 

 numbers. 



Frequently we can see the gradual or even sudden extinction 

 — generally, in some measure at least, owing to inan — of a species in 

 some of its haunts, especially of the larger and more conspicuous 

 forms, such as, in England, the wolf, the bear, and the beaver, the 

 Gripsy Moth, and the large Copper Butterfly (OJn-ysophonus dispar 

 Haw.) ; in Africa the quagga (Equnn quagga L.) ; and in North 

 America the bison and the passenger pigeon {Ectnjyistes vdgratorms L.) 



Sometimes we can see the change in the physical conditions and 

 can deduce from this, even if we cannot observe it directly, the conse- 

 quent effect upon the organisms dependent on them. This may take 

 one or more of three possible courses : — (i) The reduction in numbers 

 and final extinction of a species owing to the direct inimical effect of 

 the environment, e.g., the destruction of many plants and animals in 

 the neighbourhood of large towns as the result of poisonous emana- 

 tions, cultivation, etc.* ; (ii) The intrusion of new competitive forms 

 better suited to the external conditions, and thus the extinction of 

 sj)ecies by indirect means; (iii) or, if the process of environmental 

 change be slow enough, a possible gradual adaptation of the organism 

 to its new conditions, with the consequent production of a new 

 physiological, and therefore presumably of a new morphological — entity, 

 forma, species, variety, call it what you will. 



(To he continued.) 



* Geo. B. Walsh, " OViservations on some of the causes (lotei-Miinint,' the Survival and Kxtinc- 

 tion of Insects." Ent. Mo. Mag., 1915, pp. 226-232, 257-201. 



