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 litoral 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 
