22 ARKIV FÖR ZOOLOGI. BAND 10. N:0 12. 



islets in a sea. The biological isolation in the former case 

 may result in a physiological, or morphological divergence 

 just as well as the geographical isolation in the latter case. 

 Small individual differences or variations may thus by in- 

 breeding in the different colonies become more or less fixed 

 so that, so to say, family races originate. This is an ana- 

 logous phenoménon to the well known fact that the stock from 

 one farm-yard often is to an experienced breeder recognizable 

 from that of another, although thedistingaishingcliaracteristics 

 are of minor importance, and both belong to one and the 

 same race. A still better example, which better excludes the 

 possible selecting influence of man, is that a zoologically in- 

 terested sportsman often can recognize the roebucks from 

 one estate from those of another, or the red stags from one 

 forest from those of another, although in both cases there 

 cannot be spöken of any real racial difference. With regard 

 to the Hyraxes the isolation may be in many cases more 

 complete as these animals probably are not apt to wander 

 far from their homes. The divergence may then continue 

 and increase, and finally become so completely fixed that 

 it can be spöken about races in some cases, and subspecies 

 in others and so on. This is, I think, an acceptable expla- 

 nation of the state of affairs which is displayed by the Pro- 

 cavias of the bnicei-group, and perhaps other groups as well. 

 Mutatis mutandis the case is, according to my opinion, 

 also very similar with the Dendrohyraxes. With regard to 

 them it is the isolation from each others of the forests which 

 they inhabit, that causes the genesis of diverging races and 

 subspecies. In East Africa especialty, there are numerous 

 forests, often confined to higher or lower mountains, which 

 are completely isolated from each other by väst stretches 

 of open steppe country. These mountain-forests form biolo- 

 gical islands in a still higher degree than the rock-fortresses 

 of the Hyraxes mentioned above. They are inhabited by a 

 fauna which is entirely different from that of the surround- 

 ing steppe, and as this fauna is adapted to the forest-life, 

 it is quite confined to the forests. For many of these fo- 

 rest- resp. arboreal animals it is unnatural, or impossible to 

 cross the open land. Their ancestors have arrived to the 

 place during a period with other climatic conditions when 

 the country was more evenly covered with forest. The mem- 



