184 GEOLOGICAL DISTKIBUTION. 



that they occur in formations widely separated from those holding 

 their, nearest of kin. Whether these be really good species or not 

 may be a matter for further consideration ; but it cannot be denied 

 that their primary recognition as such is based upon the assumption 

 that no species, after it once became extinct, ever again came into 

 existence. While there is much that speaks in favor of this doc- 

 trine, it may, nevertheless, safely be asked, in what lies the proof 

 of its correctness ? Surely we possess no knowledge which will 

 permit us to state just when a given species became extinct. Dis- 

 appearance from one locality is in itself no indication of absolute 

 extinction, any more than appearance is an indication of primary 

 origination. How, then, can we ascertain, when a given form 

 supposed to be extinct reappears after an interval of a formation, 

 whether that form in reality became extinct or not ? It is usually 

 assumed that it did not, and its range in time is correspondingly 

 extended; or, the reappearing form, with no distinctive characters 

 of its own, is elevated to the rank of a new species, and the extinc- 

 tion of the first species insisted upon. But it is evident that this 

 method of forcing the point is in the nature of an argumentum in 

 circo, and leaves the question of extinction and reappearance in the 

 condition of "not proven." 



While, apart from the proof that is lacking in the matter, the 

 doctrine of non-reappearance seems to commend itself by a certain 

 '•plausibility," it may still- be doubted whether this supposed 

 plausibility is not more a matter of preconception than of actual 

 fact. If evolution is true, and there are few among scientists 

 who would deny that it is so, can it not readily be conceived that, 

 as the result of the interaction of the physical and organic forces, 

 identical forms may have been evolved as the heads of very distinct 

 lines of descent ? And, if so, may not this process have operated 

 through distinct periods of time ? Mr. Darwin, in attacking the 

 problem, thus states the case ("Origin of Species," p. 379, ed. 

 1S36): " We can clearly understand why a species, when once losl, 

 should never reappear, even if the very same conditions of life, 

 organic and inorganic, shovild recur. For though the offspring of 

 one species might be adapted (and no doubt this has occurred in 

 innumerable instances) to fill the exact jjlacc of another species in 

 the economy of nature, and thus supplant it, yet the two forms — ■ 

 the old and the nev.' — v.'ould not be identically the same, for both 



