I.] CENTRES OF EVOLUTION. 21 



rodent family Octodontida, which is now mainly confined to Africa 

 south of the Sahara, and Central and South America, the genera 

 may be less closely allied, although sufficiently so to indicate 

 a continuous distributional area, or rather a common centre of 

 dispersal, at no very remote epoch. 



Allied families, with a small number of genera, severally con- 

 fined to distant localities are likewise of the highest value in 

 building up the former history of the globe. As examples of this 

 nature may be cited the tree-shrews (Tupaiida) of the Oriental 

 countries and the jumping shrews (Macroscelidida) of Africa on the 

 one hand, and the Solenodontidce of the West India islands, and 

 the Centetidce of Madagascar on the other. Such families must 

 clearly have had a common centre of origin and dispersal ; the 

 available evidence suggesting that in the case of the two former 

 such centre was Europe. 



Although of far less common occurrence than among families 

 or genera, discontinuous distribution in an order is perhaps of even 

 more importance than either of the other cases, as it implies a 

 greater interval of time since the original dispersal took place, and, 

 therefore, carries back such conclusions as can be drawn in regard 

 to former land-connections to a still earlier epoch. Among 

 mammals the only instance of this nature is to be found in the 

 marsupials 1 , of which two families are American (and mainly 

 South and Central American), while all the others are confined to 

 Australasia and some of the adjacent Malayan islands. In this 

 case also there is abundant evidence of the wide distribution of 

 the whole group in former epochs of the earth's history. 



This last instance leads on to the consideration of what may 

 be termed "centres of evolution." In a previous 



paragraph it has been stated that, according to the " f 



available evidence, a very large proportion, if not 

 the whole, of the terrestrial mammalian life of the globe has 

 originated in the northern hemisphere, from which it has spread 

 southwards in a continuous successive series of waves. When, 

 however, certain groups of mammals had once reached the more 



1 In this work the Effodientia are separated from the Edentata; but when 

 these are united, there is a second instance. 



