ABSTRACT
The problems of the distribution of animals and plants in a country are riddles, the difficulty of which varies with the complexity of its history. In regions of great stability they are simpler than where important geographical and climatic changes have taken place in the past. Thus, on the old view of the geological uniformity of the continent of Africa, these problems might have been expected to be comparatively simple, whereas they have always proved exceptionally confused and intricate. After making a preliminary collection in East Africa, I compared notes with those of the residents, such as Mr. Ainsworth of Machakos, the late Mr. Bell Smith of Melindi, and the late Dr. Charters of Kibwezi, who had had experience on the west coast. The result sorely puzzled me, by bringing out apparently glaring contradictions in the facts of distribution. Thus certain groups run across Africa from east to west, while others extend from north to south. The commoner beetles, butterflies, and birds seemed to belong to a fauna that spread across the continent from the Atlantic to the Indian Ocean. On the other hand, some less important groups of animals, and some of the more striking of the plants, have their nearest affinities with those of
Abyssinia and the Cape. That different groups of living creatures have different geographical distributions is a wellknown fact. As shown by Dr. Blanford, in his remarkable 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 specialisation 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 connection ; when they were introduced, they were apparently able to spread in any direction, and thus their present distribution 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. According 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.
