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ing uniformity is everyvvhei-e seen in the operations of Nature, whicli 

 may warrant a presumption that the same rule will hold true here, 

 alike in the complex structures and in the more simply organized 

 plants — whether that rule shall ultimately establish or refute the idea 

 of a transition of species. I use this term " transition," to signify the 

 production of one species fi'ora another, whether it be effected by de- 

 scent, or in any other mode. And my purpose here is to point out 

 the kind of evidence, upon the validity of which a decision must be 

 made, in forming our opinions upon the matter. This evidence may 

 be conveniently arranged under three general heads : — 



1. Inferences which have been drawn from the past history of the 

 earth, and those changes in the character of its Flora which have been 

 brought to light by geological research. 



2. The tendency of species to vary ; and hence the production of 

 such intermediate and connecting links between different species, as 

 would warrant a presumption that no permanently impassable limits 

 are assigned to them. 



3. Direct facts towards establishing the transition from one species 

 into another. 



First, then, it will be conceded that many species of plants former- 

 ly flourished on the surface of the earth, which were quite distinct 

 from those now growing around us in their stead. Further, there is 

 good reason for believing that none of the present species existed in 

 those remote periods. And it seems highly probable, if not certain, 

 that past changes in the earth's Flora were effected gradually ; the 

 whole Flora of any one period not being destroyed in the aggregate, 

 to make room for another entirely different Flora; — but that species 

 after species disappeared, species after species appeared, singly and 

 successively ; no total change occurring at once, unless as a local 

 event, which would not implicate the general Flora of the earth. 



It is extremely difficult to account for these changes, by natural 

 means, unless on the hypothetical assumption that one species pro- 

 duced another, under changed conditions of climate or other circum- 

 stances. In rejecting that hypothesis, we are thrown upon the super- 

 natural alternative of assuming, quite as gratuitously, a direct and 

 oft-repeated exercise of Creative Power. But this latter assumption is 

 not consistent with anything now seen in Nature, where all seems to 

 proceed uniformly, in accordance with pre-settled laws. Still, gratu- 

 itous though it is, the supernatural alternative is the one generally 

 received by the vulgar, and admitted — tacitly, at least — by men of 

 science. The author of the ' Vestiges ' found this impediment in his 



