Iterative Formation of Species. 661 
13. ITERATIVE FORMATION OF SPECIES. 
There is a great deal of evidence to show that species 
arise in groups, and that they originate discontinuously 
in the geological strata. 1 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 geo- 
logical strata is the usual one. 2 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 separated 
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, 3 has recently pointed out the agree- 
ment of my views with his conclusions. 4 Our Fig. 149, 
(p. 655) could be used as a schematic representation of 
. 0. FOCKE, Die Pflanzenmischlinge, 1881, p. 509. 
2 E. KOKEN, Paldontologle und Descendenzlehre, Jena, 1902, and 
the literature cited here. See especially pp. 12-13. See also W. B. 
SCOTT, On Variations and Mutations, Am. Journ. Sc., Vol., XLVIII, 
P- 355- 
3 CHARLES A. WHITE, The Relation of Biology to Geological In- 
vestigation, Report of the U. S. Nat. Mus., 1892, p. 245. 
4 The same, The Saltatory Origin of Species, Bull. Torrey Bot. 
Club, Aug. 1902. 
