The Past the Key to the Present 329 



fishes were lifted over barriers by waterspouts, and there is on record 

 even an hypothetical land tortoise, full of eggs, which colonised an 

 oceanic island after a perilous sea voyage upon a tree trunk. 

 Accidents will happen, and beyond doubt many freaks of discon- 

 tinuous distribution have to be accounted for by some such means. 

 But whilst sufficient for the scanty settlers of true oceanic islands, 

 they cannot be held seriously to account for the rich fauna of a large 

 continent, over which palaeontology shows us that the immigrants 

 have passed like waves. It should also be borne in mind that there 

 is a great difference between flotsam and jetsam. A current is an 

 extension of the same medium and the animals in it may suffer no 

 change during even a long voyage, since they may be brought from 

 one literal to another where they will still be in the same or but 

 slightly altered environment. But the jetsam is in the position of a 

 passenger who has been carried off by the wrong train. Almost 

 every year some American land birds arrive at our western coasts 

 and none of them have gained a permanent footing although such 

 visits must have taken place since prehistoric times. It was there- 

 fore argued that only those groups of animals should be used for 

 locating and defining regions which were absolutely bound to the 

 soil. This method likewise gave results not reconcilable with each 

 other, even when the distribution of fossils was taken into account, 

 but it pointed to the absolute necessity of searching for former 

 land-connections regardless of their extent and the present depths 

 to which they may have sunk. 



That the key to the present distribution lies in the past had 

 been felt long ago, but at last it was appreciated that the various 

 classes of animals and plants have appeared in successive geological 

 epochs and also at many places remote from each other. The key to 

 the distribution of any group lies in the configuration of land and 

 water of that epoch in which it made its first appearance. Although 

 this sounds like a platitude, it has frequently been ignored. If, for 

 argument's sake, Amphibia were evolved somewhere upon the great 

 southern land-mass of Carboniferous times (supposed by some to have 

 stretched from South America across Africa to Australia), the dis- 

 tribution of this developing class must have proceeded upon lines 

 altogether different from that of the mammals which dated perhaps 

 from lower Triassic times, when the old south continental belt was 

 already broken up. The broad lines of this distribution could never 

 coincide with that of the other, older class, no matter whether the 

 original mammalian centre was in the Afro-Indian, Australian, or 

 Brazilian portion. If all the various groups of animals had come into 

 existence at the same time and at the same place, then it would be 

 possible, with sufficient geological data, to construct a map showing 



