363 M. L. Agassiz on the Primitive Diversity and 



genera and species; as if both genera and species had not a 

 natural existence, independent of the estimates of naturahsts. 

 It would be just as reasonable for astronomers to complain of 

 the great number of stars, as for geologists to object to the in- 

 vestigations of zoologists, on the ground that they lead to the 

 " making " of " too many species.^' 



The difficulty with reference to the identification of species is 

 threefold: 1. different species may be considered as identical; 

 2. specimens of the same species in different states of preservation, 

 or of different age, or sex, &c., may be considered as distinct spe- 

 cies ; or 3. the same species may have been described by different 

 authors under different names, and their identity afterwards over- 

 looked by later writers. Who does not see what amount of error 

 may accrue from the indiscriminate use of materials which are 

 not first submitted to a very critical revision in these different 

 respects, not to speak of the general difficulty of agreeing upon 

 the limits of specific differences ? With regard to this last point, 

 however, I would say, that any one who in discussing general 

 questions would only use materials revised candidly with the 

 same principles, could not fail to obtain at least uniform results. 

 And when the results of investigations made upon materials cor- 

 rected in different ways by different authors are compared with 

 one another, if these differences are kept in view, the disagree- 

 ment in the results would not be found so great as it might other- 

 wise seem. The astronomers and physicists have long learned 

 to correct their observations before using them, and to take into 

 consideration what they call the personal equation of different 

 observers ; — are we never to learn from them a lesson in the esti- 

 mation of our respective investigations, and shall our facts for 

 ever be used without being first corrected for all the possible 

 causes of error and disagreement ? As long as there are differ- 

 ences of opinion respecting the natural limits of genera and spe- 

 cies, is it not absolutely necessary to reduce or expand the scale 

 applied to the investigations of different authors, when using 

 them for the same purposes, exactly in the same manner as ther- 

 mometric observations made with the scales of Reaumur or Cel- 

 sius or Fahrenheit are reduced to the same standard, before 

 being compared. 



In the second place, species must be referred to genera circum- 

 scribed within the same limits, before they can fairly be compared, 

 or at least lead to trustworthy general results. As long as cer- 

 tain bivalve shells of the carboniferous and oolitic series were 

 referred to the genus Unio, it could appear that the family of 

 Naiades began its existence at a very early period ; but since the 

 oolitic species of this kind have been ascertained to differ essen- 

 tially from our freshwater shells, and to constitute by themselves 



