238 EAST AFRICAN FLORA AND FAUNA part in 



Abyssinia and the Cape. That different groups of living 

 creatures have different geographical distributions is a well- 

 known fact. As shown by Dr. Blanford, in his remark- 

 able presidential address to the Geological Society in 1890, it 

 can be easily explained by the assumption, that the distribution 

 of land and water has varied greatly at different periods in 

 geological history. A group of animals, therefore, that made its 

 appearance on the earth at one period, was able to spread along 

 very different lines from those followed by a later group, when 

 old land-masses had been broken up, and seas once connected 

 had become separate. Thus, if we compare the distribution of 

 different groups of animals, we find a gradually increasing special- 

 isation as we pass from the oldest to the youngest groups. 

 This is shown by the accompanying four sketch-maps. The 

 oldest of the five classes of vertebrate animals is that of the fish, 

 of which only the fresh-water forms are of any value in this con- 

 nection ; when they were introduced, they were apparently able 

 to spread in any difection, and thus their present distribu- 

 tion appears to be determined mainly by temperature, for the 

 faunas range round the world in three bands. By the time 

 the tortoises appeared in the period of the Trias, or New Red 

 Sandstone, the land of the southern hemisphere had apparently 

 been broken up, and it was only to the north of the equator 

 that the animals were able to range round the globe. Accord- 

 ing to the tortoises, therefore, North America, Europe, and 

 Asia are all part of the same province ; but before the 

 introduction of the lizards, remains of which first occur in 

 the Purbeck limestones, great geographical changes had 

 occurred. These reptiles could not spread westward into 

 North America, but they made up for this restriction by 

 extending southward throughout Africa. After another great 

 lapse of time, snakes appeared upon the scene in the age of 

 the London Clay (a part of the Lower Eocene) ; the European 

 species were now cut off from Africa, and in Asia were limited 

 to the western half of the continent. The passerine birds, on 

 which Dr. Sclater's classification is mainly based, soon followed 

 the snakes ; they were cut off from America and Tropical 

 Africa, but their powers of flight enabled them to spread over 

 the whole of Europe and Asia, though they did not succeed in 

 entering India, Siam, and the Malay Peninsula. 



