A HISTORY OF SCIENCE 



" What kind of proofs, therefore, could we reasonably 

 expect to find of the origin at a particular period of a 

 new species? 



" Perhaps, it may be said in reply, that within the 

 last two or three centuries some forest tree or new 

 quadruped might have been observed to appear sud- 

 denly in those parts of England or France which had 

 been most thoroughly investigated that naturalists 

 might have been able to show that no such being in- 

 habited any other region of the globe, and that there 

 was no tradition of anything similar having been 

 observed in the district where it had made its appear- 

 ance. 



" Now, although this objection may seem plausible, 

 yet its force will be found to depend entirely on the 

 rate of fluctuation which we suppose to prevail in the 

 animal world, and on the proportions which such con- 

 spicuous subjects of the animal and vegetable king- 

 doms bear to those which are less known and escape 

 our observation. There are perhaps more than a mill- 

 ion species of plants and animals, exclusive of the 

 microscopic and infusory animalcules, now inhabiting 

 the terraqueous globe, so that if only one of these were 

 to become extinct annually, and one new one were to 

 be every year called into being, much more than a 

 million of years might be required to bring about a 

 complete revolution of organic life. 



" I am not hazarding at present any hypothesis as to 

 the probable rate of change, but none will deny that 

 when the annual birth and the annual death of one 

 species on the globe is proposed as a mere speculation, 

 this, at least, is to imagine no slight degree of instability 



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