18 NATURE SKETCHES IN TEMPERATE AMERICA 



tinuous; that is, variations occurring irregularly, mostly large 

 and comparatively rarely, and small abundant variations 

 occurring in gradual series. Among the former are to be 

 ranked the occasional sports and monsters familiar to breeders, 

 while in the latter, Darwin believed himself to have at hand 

 the necessary, ever-present materials to serve natural selection 

 as a basis for species transformation. Hence the slight, but 

 abundant and ever-present, fluctuating, continuous variations 

 are often called "Darwdnian variations." 



The law of Quetelet applies solely to the Darwinian variations. 

 The law is "that these variations occur according to the law 

 of probabihties (or law of error); that is, that the shghtest 

 variations away from the modal or average type will be the 

 most abundant, and that the number of varying individuals 

 will be progressively less the farther away from the modal 

 type the variations of these individuals are." 



DeVries maintained that species have arisen without the 

 aid of natural selection; the actual production of species 

 being due to saltation or mutation quite in contrast with 

 the gradual transmutation of Darwin.^ This conception of 

 species formation is different from the theories of Nageli, 

 Eimer, and Cope, who maintained that non-fortuitous and 

 determinate variations were determined by certain causes 

 inherent in life (Nageli), or causes extrinsic to Hfe, but 

 imposed upon it (Eimer). 



Recently Johannsen has endeavored to interpret by experi- 

 mental investigation those slight variations that seem to be 

 independent of environment, which the biometric students 

 call "frequency polygon." Davenport,^ in reviewing the work 

 of Johannsen, says: 



"The biometric 'school' laid stress on this sort of variation 

 and held that by selective breeding from the extreme variants 

 through many generations an indefinitely wide departure 

 from a starting point might be effected. This DeVries denied, 

 but held that, while such selection might lead to a certain 

 departure from the mode, the degree of such a departure 

 was restricted through a strong regressive tendency. Here 



■ Natural selection determines the future existence of the form in either casr. 

 » Science, N. S., XXX. 1909. p. 852. 



