190 Mr. A. R. Wallace on the Law which has regulated 



fur^iish a countless number of analogous examples;— the Go- 

 liathi of Africa, the Ornithopterce of the Indian islands, the Heli- 

 conidce of South America, the Danaidce of the East, and in all, 

 the most closely allied species found in geographical proximity. 

 The question forces itself upon every thinking mind, — why are 

 these things so ? They could not be as they are, had no law re- 

 gulated their creation and dispersion. The law here enunciated 

 not merely explains, but necessitates the facts we see to exist, 

 while the vast and long-continued geological changes of the 

 earth readily account for the exceptions and apparent discre- 

 pancies 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 test 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 connect- 

 ing facts which exist in nature, he expects facts alone to be 

 brought to disprove it ; not a-priori arguments against its pro- 

 bability. ,, 



The phaenomena of geological distribution are exactly analo- 

 gous to those of geography. Closely allied species are found 

 associated in the same beds, and the change from species to spe- 

 cies appears to have been as gradual in time as in space. Geo- 

 logy, however, furnishes us with positive proof of the extinction 

 and production of species, though it does not inform us how 

 either has taken place. The extinction of species, however^ 

 offers but little difficulty, and the modus operandi has been well 

 illustrated by Sir C. Lyell in his admirable ^ Principles.^ Geo- 

 logical changes, however gradual, must occasionally have modi- 

 fied external conditions to such an extent as to have rendered 

 the existence of certain species impossible. The extinction 

 would in most cases be effected by a gradual dying-out, but in 

 some instances there might have been a sudden destruction of a 

 species of limited range. 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 have from time to time been replaced by new ones down to the 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 eliminate 

 from known facts a law which has determined, to a certain de- 

 gree, 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. 



Much discussion has of late years taken place on the question, 

 whether the succession of life upon the globe has been from a 

 lower to a higher degree of organization '{ The admitted facts 

 seem to show that there has been a general, but not a detailed 

 progression. Mollusca and Radiata existed before Vertebrata, 



