E VIDENCES OF EVOLU TION. 93 



rather narrow limits, and, so far as we can retrace 

 antiquity, we perceive that these limits were the 

 same as at the present. We are thus obliged to ad- 

 mit of certain forms which, since the origin of things, 

 have been perpetuated, without exceeding these 

 limits ; and all the beings appertaining to one of these 

 forms constitute what is termed a species. Genera- 

 tion being the only means of ascertaining the limits 

 to which varieties may extend, species should be 

 defined as the reunion of individuals descended from 

 one another, or from common parents, or from such 

 as resemble them as closely as they resemble each 

 other; but although this definition is rigorous, it will 

 be seen that its application to particular individuals 

 may be very different when the necessary experi- 

 ments have been made." 



But not only, according to Cuvier, are existing 

 species fixed and the result of special creative ac- 

 tion ; the same views must also be held regarding 

 the countless geological species which have so long 

 disappeared from the face of the earth. The great 

 naturalist was a firm believer in the doctrine of suc- 

 cessive creations and destructions, of a series of de- 

 populatings and repeoplings of the world. As is 

 well known, he was the author of the celebrated 

 Period or Concordistic theory, which attempts 

 to reconcile the statements of the Mosaic narra- 

 tive of creation with the declarations of geology 

 and paleontology a theory which has had a 

 great vogue, and which, after the lapse of three- 

 quarters of a century, has even now not a few advo- 

 cates. 



