26 PHYSICAL INVESTIGATION. 



there may not have been originally distinct parallel stocks 

 the same species, which in their essential character, as well as 

 in respect of the range of variation through which they pass, 

 may exhibit an unlimited fecundity between each other. 



However decidedly we may oppose a theory of the origination 

 of new species by the production of hybrids, this much must 

 be admitted, that from unlimited prolificacy alone the unity of 

 species can hardly be inferred. On the other hand we can 

 scarcely agree with Holland, 1 when he asserts that the theory 

 that individuals, however much they may differ, belong to one 

 species if they prove to be of unlimited prolificacy, moves in a 

 circle, and assumes what remains to be proved : for it is an 

 empirical fact that a really unlimited prolificacy nowhere oc- 

 curs where important differences of organization prevail ; and 

 an essential feature of the character of fecundity as a mark of 

 species lies in this, that it does not involve the unsolvable doubt 

 of common or separate descent, but ignores it. 



Notwithstanding the variety of objections which may be 

 raised against unlimited fecundity as a decisive character of 

 species, it cannot, as has recently been said, 2 be considered as 

 of merely secondary importance. It retains its importance, 

 though it is not ascertained how many generations have 

 proved prolific in order to arrive at the conclusion that they 

 belong to the same species ; and further, sterility not only 

 occurs between individuals of the same stock, but the extinc- 

 tion of some races is as clearly demonstrated as the extinction 

 of whole species. If recent experience has shown in the 

 breeding of domestic animals that different races are some- 



1 " De FHomine et des Races Humaines/' p. 213, 1853. 



2 Giebel (loc. cit.) has misrepresented this point, by stating, what no one 

 has asserted, that fecundity has been considered as the only criterion of spe- 

 cies ; hence he requires from the adherents to this doctrine that they should 

 only count such individuals to be of one species whose unlimited fecundity is 

 experimentally proved. It is a further misrepresentation, when he says, that 

 difference of species cannot be inferred from sterility, for however correct 

 this may be, it does not follow that fecundity should be neglected as a cri- 

 terion ; for it is only contended that, where minor differences of organization 

 exist, the specific nature of which is doubtful, sterility or prolificness may 

 afford important assistance in deciding the question; though it may be 

 granted that the decision obtained in this way is not absolute and final, it is 

 certainly not valueless and unimportant, unless the capacity of reproduction 

 is not considered as an essential character of the animal world. 



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