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give plausibility to the supposition that varieties do not always revert 

 to the central types of the species from which they originated. 



5thly. — A variety ( if such there be ) in which the tendency to re- 

 produce its own like has superseded the tendency to revert to the 

 central type of the original species, would possess the essential 

 character of a species in itself,— namely, its own distinct and per- 

 manent central type. It has not yet been proved that any such variety 

 exists, neither can it be disproved. 



6thly. — The discordant opinions of botanists, as to which plants are 

 species and which are varieties only — the occurrence of varieties in- 

 termediate between presumed species — the power of changing from 

 the central type into varieties, and back again to the central type — 

 the tendency of some varieties to become hereditary, probably in obe- 

 dience to the law of ' like producing like ' — with other facts, point to- 

 wards the conclusion that varieties may gradually become species ; 

 although these facts are far from sufficient to establish that conclusion. 



On the whole, therefore, we seem to be justified in asserting, that 

 our knowledge of the present events in nature, taken by itself, should 

 incline us to a conclusion which is directly adverse to the theory of 

 " progressive development " or " transition of species ;" yet without 

 affording us any actual disproof of that theory. 



It is otherwise when our range of thought embraces the vastly 

 wider space of time, the events of which are investigated by geolo- 

 gists. There we find ample evidence to justify the conclusion that 

 different species succeeded to each other. And no better mode of ac- 

 counting for this succession has been suggested, than the hypothesis 

 that one species passed into another, under changing external condi- 

 tions. Supposing this transition of species to have taken place very 

 gradually, and through a very long series of descents, it would not re- 

 quire more rapid change ( from central types into varieties, and from 

 a less variety into a greater ) than we see actually occurring in the 

 production of varieties at the present period of the earth's history. 



Could we ascertain that some varieties will continue to vary from 

 their central type, through many successive descents ; and that, as 

 they become less similar to their original central type, the tendency 

 of " like to produce like " will overpower and supersede the tendency 

 to revert to the original type ; — in this case, we might hold the " tran- 

 sition of species " to be a theory founded on facts. At present, it is 

 scarcely more than a plausible hypothesis, invented to account for 

 facts, and accounting for them better than any other hypothetical 

 suggestion has done. Hewett C. Watson. 



Thames Ditlon, July 2, 1845. 



