3(b ZOOLOGY 



wholly explained by its large fecunditj' ; for although equally 

 reproductive in Europe it increases less rapidly there than 

 here. Also it is not due to any peculiarity of our countr}', for 

 the Ijird is a similar pest in Australia. Similar facts concern- 

 ing the spread of other animals lead us to c(jnclude that it is 

 the new countrj- which permits tlie rapid spread and conse- 

 C|uent destructiveness. Thus, when the caltbage-butterfly 

 (Picn's rapcc) was brought to this country, it spread with such 

 rapidity that, starting in 1860 at Quebec, it has now spread all 

 over the United States as far as the Rockj' ^fountains. Again, 

 the grapevine insect pest. Phylloxera, a native of this countr}-, 

 but not particularh' destructive here, has been accidentallj^ 

 transported to France, and there it has wrought great havoc 

 in the vinej'ards. Another instance, this time of an aquatic 

 animal, shows the same result : the periwinkle, Littorina 

 littoria,^ now the commonest snail on the seashore north of New 

 York, has migrated down the shore from Halifax since 1868. 

 This old species in the new country' has almost driven out the 

 other shore mollusks, to such an extraordinarj- degree has it 

 multii)lied. Now wh}- should animals in a new country de^-elop 

 with such unusual rapidit}-? It is because, coming into a new 

 country, they ha\'e left Ijehind them their natiu-al enemies, and 

 there has not yet been time for them to accjuire new ones. 

 E^fentually new enemies are gained, or their old ones over- 

 take them, and then the numbers of the exotic form become 

 reduced; a new equilibrium Ijecomes established. 



As an example of the structure of a bird, and for comjiarison 

 with the anatomj' of the English sparrow, the common pigeon 

 may be taken. 



General Form of the Body. — If the smaller (or " contour ") 



1 See page 216. 



