294 GEOLOGY. 



the aspect of present forms that the classes and some orders are readily 

 recognized. 



The concrete question then arises, How does the initiation and 

 the divergence of the structures and of the types that preceded the 

 Cambrian stage compare with the developments since? The answers 

 given by expert students of life-development very naturally vary, but, 

 so far as known to us, they all assign a decidedly greater value to the 

 pre-Cambrian than to the post-Cambrian evolution. Formulated in 

 numerical terms, from sixty to ninety per cent, of the whole evolution 

 is attributed to pre-Cambrian times. 



The Succession of Faunas. 



Under the doctrine of evolution, it is presumed that the life of 

 every past stage has grown out of that which immediately preceded 

 it, and that it has merged into that which immediately followed it. 

 It is usually assumed that if no exceptional influences affected the 

 process, there was a continuous series of slow changes without sharp 

 lines of demarkation. If this conception were realized in fact, it would 

 be less appropriate to speak of a succession of faunas than of one con- 

 tinuous ever-changing fauna. It is not yet demonstrated, however, 

 that evolution proceeded solely by very slight changes coming in from 

 generation to generation. A doctrine of evolution by distinct and 

 abrupt mutations has recently been advanced by DeVries. 1 This 

 maintains that changes as great as those usually regarded as distinguish- 

 ing species may take place between parent and offspring, and that 

 the new characters so introduced may be perpetuated and remain 

 permanent. This is equivalent to maintaining that new species may 

 arise abruptly when the parent form is in what is termed the muta- 

 ting stage. . It is not held that these changes take place in all species 

 at all stages, but only in some species at certain periods of their exist- 

 ence when they are in this mutating condition. The doctrine, as 

 at present held, does not maintain that a whole fauna would be likely 

 to change into a different fauna abruptly, but merely that new species 

 might arise in it abruptly. At present the doctrine rests chiefly on 

 observations and experiments made on a few plants. It remains 



1 Die Mutationstheorie, 1903. See also Bateson's Materials for the Study of Varia- 

 tion, 1894; and W. B. Scott, On Variations and Mutations, Am. Jour. Sci., 1894, p. 

 355. 



