268 OKTMANN— DISTRIBUTION OF DECAPODS [April 3 r 



the difference of the means of dispersal of the various groups of 

 animals. On account of these anomalies Wallace constructed his 

 regions chiefly for Mammals and Birds, excluding all the rest of the 

 animal kingdom. 1 



This method, however, can never be satisfactory. It amounts to 

 nothing but the creation of an arbitrary scheme which may corre- 

 spond to some of the facts; but if there are any other facts that do 

 not fit into it — as very often happens — they are simply thrown out 

 and neglected. 



But this is not all. Even the restriction of Wallace's regions to 

 a single group of animals proved insufficient to cover all cases 

 within this group. This is true also of all other schemes that have 

 been proposed by other writers for the same or other smaller 

 groups. In every single instance there were exceptions to the rule, 

 and for some time it seemed difficult or even impossible to deal 

 with these apparent anomalies; in fact, none of the proposed 

 divisions into regions can be applied to all cases, even within 

 smaller groups. 



The correct understanding of this fact, that a large number of 

 animals does not submit to any of the proposed schemes that profess 

 to comply with the present distribution of the condition of life, was 

 made possible by the consideration that the actual distribution of 

 any animal must have originated in the past. Although there are 

 some animals the history of which does not go very far back, in a 

 geological sense, there are others which do, and, generally speak- 

 ing, we may say that the farther back we go in geological history 

 the more different were the conditions of life from what they are 

 now, and the present distribution of the respective forms must nec- 

 essarily appear the more strange and anomalous. Wallace, indeed, 

 tried to remove this difficulty in a very peculiar way. He simply 

 propounded his principle of the permanency of the continents, 

 which means to say that the present distribution of land and water 

 (and in general of the physical conditions of life) did not change 

 materially during the earth's history, and that the external features 

 of the earth's surface have remained practically identical from time 

 immemorial up to the present. That this principle is without 



1 This exclusive restriction to the higher forms of life (Mammals, Birds) is a 

 principle of Wallace and has been expressly maintained by him as late as in 1894 

 (see Nature, Vol. xlix, 1894, p. 610). 



