EVOLUTION AND DISTRIBUTION OF ANIMALS 279 



and where the general physical conditions are favourable. 

 The fact that the family Centetidse consists of six very 

 distinct genera (five in Madagascar and one in the 

 Antilles) is a sufficient indication that it was once an 

 extensive group. In the Lower Miocene of Auvergne, the 

 fossil remains of a small animal have been found, which is 

 provisionally classed in this very family ; and both in 

 Europe and America a considerable number of the remains 

 of Insectivora of peculiar genera have been discovered, 

 indicating that this order of mammals is a very ancient 

 one, which probably long ago arrived at its maximum of 

 development, and has been diminishing in proportion as 

 the larger and more perfectly organised forms have been 

 increasing. It is interesting to note that the two localities 

 where the Centetidae still linger have many remarkable 

 similarities and correspondences. Both are insular groups 

 of the first rank ; both are separated from their adjacent 

 continents by very deep sea ; both are situated just within 

 the line of the tropic; both are subject to hurricanes; 

 both are very mountainous; in both all the higher 

 mammalia are very deficient ; and the differences of their 

 forms of life from those of the adjacent lands are such as 

 to indicate that they have both remained insulated for a 

 considerable period geologically. There can be little 

 doubt that these resemblances have something to do with 

 the continued existence in both of isolated members of a 

 once widespread group of mammals, of a comparatively 

 low type of organisation, and unable to bear the com- 

 petition to which they have been exposed in continental 

 areas. The same principles will, of course, explain the 

 presence in Madagascar of a mouse allied to an American 

 group, of three American genera of colubrine snakes, and 

 of lizards belonging to the peculiar American family 

 Iguanidse, as well as of the beautiful green diurnal moths 

 of the genus Urania, and several beetles of decidedly 

 South American affinities. In some other cases we have, 

 as it were, a relic of the former wide extension of now re- 

 stricted groups. Thus, one genus of snakes, Ahsetulla, is 

 found in Africa and South America as well as in Mada- 

 gascar ; while a genus of geckoes, Phyllodactylus, inhabits 



