Iterative Formation of Species. 661 



§ 13. ITERATIVE EORMATION OE SPECIES. 



There is a great deal of evidence to show that species 

 arise in groups, and that they originate discontinuously 

 in the geological strata.^ For various groups of animals 

 and plants the exhaustive studies of Koken have shown 

 that this mode of the origin of new forms in the gc(j- 

 logical strata is the usual one.- He calls this phenomenon 

 the iterative formation of species. According to him a 

 persistent species produces ''varieties" which appear in 

 swarms at certain periods ; these periods are sej^arated 

 by more or less long phases of rest. He observed this 

 first among the more ancient gastropods; but cases of the 

 iterative formation of species have been described also 

 amongst the Craniadae and Pectinidae. 



It does not seem to me to be going too far to argue 

 that the conclusions derived, in the foregoing section, 

 from the actual observation of the process of mutation 

 fit in with these results of paleontological investigation 

 in a perfectly simple and satisfactory way, whilst the 

 old theory of selection can only account for this perio- 

 dicity by the help of special hypotheses. White, who has 

 thorougly investigated these phenomena from a paleonto- 

 logical point of view,^ has recently pointed out the agree- 

 ment of my views with his conclusions.'* Our Fig. 140, 

 (p. 655) could be used as a schematic representation of 



^ W. 0. FocKE, Die PHanzenmischlingc, 1881, p. 509. 



^ E. Koken, Pal'dontoloiiic nnd Dcsccndcnzlchrc, Jena, TQ02. and 

 the literature cited here. See especially pp. 12-13. See als(i W. !'.. 

 Scott. On Variations and Mutations, Am. Journ. Sc, Vol.. XLVIIi, 



P- 355- 



■''Charles A. White. The Relation of Biology to Geological In- 

 vestigation, Report of the U. S. Nat. IMiis.. 1892. p. 245. 



* The same. The Saltatory Origin of Species. Bull. Torrcy Bot. 

 Club, Aug. 1902. 



