MODE OF ORIGIN OF A FAUNA. 447 



province from India, but it was not always so ; for the Stegodon 

 clifti, Stegodon insignis, Elephas namadicus, and Elephas primi- 

 genius, which are fossils found in the Siwalik and Narbadda deposits, 

 also occur in Japan, presumably of the same age. 1 



The Siwalik fossils comprise far more African types than are found 

 living in India, and they also include many genera which are met with 

 in the older Tertiary rocks of Europe. And just as we are entitled to 

 infer that such genera as Hyopotamus and Anthracotherium and Ace- 

 rotherium. which occur in the Miocene of Europe, migrated eastward 

 to from part of the Siwalik fauna, so we may conclude that the giraffe 

 and ostrich, which were Siwalik types of life in the Pliocene period, 

 migrated from India to Africa, and therefore we must look to this 

 ancient geological connection for an explanation of the partial com- 

 munity of life between the existing Ethiopian and Oriental regions. 

 There is no element in a fauna, so far as we are aware, in any period 

 of geological time which does not prove this migration to have been 

 always in progress. 



Origin of Faunas. Suppose, by way of hypothesis, that we have 

 to deal with a world in which the surface may be divided into natural 

 history provinces, denned like the spaces on a chess-board, where 

 the value of the moves of the several pieces is different, and that 

 after the life has accumulated on a given area, K, upheaval affects 

 the sea-bed of an adjacent region along a line A, B. The result of 

 this upheaval is to displace the life from the region S, which adjoined 

 the line A, B, and cause it to migrate to the region R ; it thus becomes 

 mixed with the life in B. But since the sea-bed at R has itself 

 undergone change, some of its life has migrated away, some species 

 have been exterminated by the altered circumstances, and similarly 

 the immigrating group loses some of the forms which were character- 

 istic of it when occupying the region S. These losses may be desig- 

 nated X. Hence a new fauna is compounded out of pre-existing 

 groups, and does not correspond with that which would be obtained 

 by simply mixing them. And just as some species of plants and 

 animals taken to new regions increase to an unheard-of extent, 

 as the Anacharis increased in our own country, the thistle in New 

 Zealand, the rabbit in South Australia, so in the history of life every 

 province becomes enriched by species which were previously unknown, 

 unimportant, or rare, that under their new geographical conditions 

 come to be dominant in number of individuals, and often in variation. 

 This gain to the province may be termed Y. Similarly, if a change 

 in the amount of elevation of the axis A, B, at one end of the axis 

 towards A, comes to lay bare the sea-bed in the region T, and causes 

 the life of this region to be superimposed upon the spot where the life 

 R was fossilised, and where the life R + S - X + Y was superimposed 

 upon it, then a third fauna will succeed, which may differ from the 

 second as much as the second differs from the first, which must have 

 been compounded out of older groups, and must itself undergo the 

 loss indicated by X, and acquire the facies indicated by Y. In this 

 1 R. Lydekker : " Palseontologia Indica." 



