ioo Animal Life and Intelligence. 



the slow-moving molluscs in adjoining valleys ; none at all 

 between those at any distance apart. 



But even if there are no well-marked physical barriers, 

 the members of a species on a continent or large island 

 tend to fall into local groups, between which, unless the 

 animal be of a widely ranging habit, there will be little 

 intercrossing. Hence local varieties are apt to occur, and 

 varieties show the first beginnings of that divergence which, 

 if carried further and more deeply ingrained, results in the 

 differentiation of species. Geographically, therefore, we 

 may have either complete isolation or local segregation, 

 and in both cases the possibility of divergence. 



Another mode of segregation arises also out of 

 geographical conditions. If variations of habits occur 

 (and structure is closely correlated with habit) such that 

 certain individuals take to the mountains, others to the 

 plains or valleys ; or that certain individuals take to the 

 forests, others to the open country; the probabilities are 

 that the forest forms will interbreed frequently with each 

 other, but seldom with those in the open, and so with the 

 other varieties. The conditions of forest life or mountain 

 life being thus similar throughout a large area, and life 

 being through elimination slowly but surely adapted to its 

 environment, there might thus arise two distinct varieties 

 scattered throughout the length and breadth of the area, 

 the one inhabiting the mountains, the other the forests. 

 In illustration of this mode of segregation, we may take 

 the case of two species of rats which have recently been 

 found by Mr. C. M. Woodford on one of the Solomon 

 Islands. These two quite distinct species are regarded by 

 Mr. Oldfield Thomas as slightly modified descendants of 

 one parent species, the modifications resulting from the 

 fact that of this original species some individuals have 

 adopted a terrestrial, others an arboreal life, and their 

 respective descendants have been modified accordingly. 

 Thus Mus rex lives in trees, has broad foot-pads, and a 

 long rasp-like, probably semi-prehensile, tail ; while Mus 

 imperator lives on the ground, has smaller pads, and a 



