PROLIFICACY. -REVERSION. 27 



times not indefinitely prolific, or produce malformed, defec- 

 tively organized young parallels of which are found in the 

 intermixture of different human races the objection is not of 

 any weight against unlimited prolificacy as a specific character; 

 for whenever this is held out as a criterion it is not asserted 

 that it occurs without exception among all individuals and 

 races of the same species, but expresses only the fact of a 

 merely limited prolificacy between individuals of specifically 

 different types. It may, however, be considered as an un- 

 avoidable defect in this criterion, that it cannot decide whether 

 there be within the same species varieties which, between 

 themselves, possess only a limited prolificacy, or none at all. 



On casting a retrospective glance at our investigations we 

 arrive at the important proposition, that inferences from com- 

 mon descent to unity of species have an absolute certainty, 

 those from unlimited fecundity have a high degree of proba- 

 bility, whilst the conclusions as to differences of species from 

 separate descent or limited prolificacy are less safe. 



A further mark of distinction between race and species is 

 also afforded by the so-called reversion; that is to say, by the 

 return of individuals prolific between themselves to the original 

 type of the parent stock, which thus proves itself permanent. 

 As the hybrids perish, the mere varieties revert, under certain 

 circumstances, to their original types, and thus show that they 

 have no specific existence. In cases of mongrels of two doubt- 

 ful types, though proving indefinitely prolific (e. g. Mulattoes 

 or Mestizoes), should they, by continued intermixture with 

 each other, return to one of the parent stocks (Negro, white, 

 or American), one would feel inclined to assume a difference of 

 species of the latter, because the transformation into a different 

 type failed ; we should then have obtained a better definition 

 of one of the criteria, fecundity ; but such a case seems not yet 

 to have occurred. There would still remain some doubt as to 

 the correctness of the conclusion, whether new characters 

 arising in the course of time may not under circumstances 

 become so fixed as to acquire a permanence equal to specific 

 characters, though it is a probable, but by no means proved, 

 supposition that all characters arising in the course of time 



