23 



are generally somewhat local, carrying out the rule which we have already seen to 

 obtain in the other classes of animal life. 



With regard to the diffusion of insects, no doubt migration plays a very 

 considerable part ; for although the majority of insects are not migratory, yet 

 when their numbers increase to any great extent, from some cause or other, they 

 are known to migrate in enormeus numbers. 



This has been known to occur with some butterflies (Vanessa Cardai), and 

 with many kinds of Orthoptera. Involuntary migration caused by strong winds 

 has frequently beep known to occur, and insects have been met with far out at 

 sea carried by the wind. 



Floods of rivers also carrying away plants with insects on them are another 

 means of dispersaL Few animals are so dependent on temperature as insects, and 

 accordingly it is found that a lofty range of snow-clad mountains is a more effect- 

 ive barrier against diffusion than a wide expanse of water ; as a proof of this, 

 entomologists inform us that the species of insects on the West and East sides 

 of the Andes in South America are mostly distinct. Another effective means of 

 diffusal is by the eggs of insects being carried off in the wool, hair, or feathers 

 of the vertebrate animals, and also in their stomachs, for they are often able to 

 resist the digestive action of the gastric juice. 



Such being the ample means of dispersal afforded to the insect class, it is only 

 surprising that their distribution should not be more general, but these instances 

 seem to me to strengthen the belief in centres of creation, and radiation therefrom 

 by natural means. 



A most curious instance of an insect confined to a narrow locality is the tsetse 

 fly of South Africa. 



With this brief notice of the invertebrate classes I must pass on to the 

 vertebrate, which for our present purpose are more important because better 

 known. 



In the Vertebrate Classes, namely, the mammals, birds, reptiles, and fishes, 

 the locomotive powers of the adult are in most cases so perfect that migration can 

 be easily effected, so long as no insuperable barrier occurs to prevent further 

 progress. The barriers which chiefly obstruct the paths of Terrestrial Vertebrates 

 are the sea, rivers, high mountains, and generally differences of climate ; and with 

 the aquatic are the land and the tempei-ature of the water. 



I have already indicated that the distribution of Vertebrate Species is 

 generally very local, and that the more remote are two countries, the more 

 different are their respective faunas, and that although the conditions of life are 

 often almost the same, yet the faunas of remote areas differ from one another ; 

 also that continuous or unbroken areas have a community of species. These 

 facts seem to point to a natural dispersal or migration from a common centre 

 or centres, and therefore it behoves those who vrish to establish the above con. 

 elusions to be able to point out the means of the general and usual migration 

 of animals, and to give some sort of explanation of the apparent exceptions to the 

 rules before stated. 



