Alfred Russel Wallace 



effect of volcanic and other action as affecting the dis- 

 tribution of species, and the exact form in which they are 

 found, even fishes giving '^ evidence of a similar kind : each 

 great river [having] its peculiar genera, and in more exten- 

 sive genera its groups of closely allied species.'' 



After stating a number of practical examples he con- 

 tinues : 



The question forces itself upon every thinking mind — 

 Why are these things so ? They could not be as they are, 

 had no law regulated their creation and dispersion. The 

 law here enunciated not merely explains, but necessitates 

 the facts we see to exist, w^hile the vast and long-continued 

 geological changes of the earth readily account for the ex- 

 ceptions and apparent discrepancies that here and there 

 occur. The writer's object in putting forward his views 

 in the present imperfect manner is to submit them to the 

 .^ tests of other minds, and to be made aware of all the facts 

 supposed to be inconsistent with them. As his hypothesis 

 is one which claims acceptance solely as explaining and 

 connecting facts which exist in nature, he expects facts 

 alone to be brought forward to disprove it, not a priori 

 arguments against its probability. 



He then refers to some of the geological '' principles " 

 expounded by Sir Charles Lyell on the ^' extinction of 

 species," and follows this up by saying : 



To discover how the extinct species have from time to 

 time been replaced by new ones down to the very latest 

 geological period, is the most difficult, and at the same 

 time the most interesting, problem in the natural history 

 of the earth. The present inquiry, which seeks to elimi- 

 nate from known facts a law which has determined, to a 

 certain degree, what species could and did appear at a 

 given epoch, may, it is hoped, be considered as one step 

 in the right direction towards a complete solution of it. 

 . . . Admitted facts seem to show ... a general, but not 

 a detailed progression. ... It is, however, by no means 



