140 



THE ORIGIN AND EVOLUTION OF LIFE 



gradual evolution of adaptive form is directly contrary to 

 Darwin's theoretic principle of the selection of chance varia- 

 tions. It is unfortunate that the same term, mutation, was 

 chosen by the botanist, Hugo de Vries, in 1901, to express his 

 observation that certain characters in plants arise by sudden 



MULTIPLICATUS 



U3) '" 



i*^^ 



m 



mW 



V, 



212 



(347) 



°*'y*^* 



(21) 



^^#^^ 



(177) 

 COLLECTIVART-SPIRIFER MUCRONATUS 



THOWBRlCXiE MILLS 



ALPtNA UMESTONE 



SOUMERVILLE UMESTONE 



EUCAJONCLAy 



SUNNYSIOE UMESTONE 



MtDOLE LAKE SHAL£ 



Fig. 36. Successive Mutations of Spirifer mucronalns. 



Specimens from the geologic section at Alpena, Mich., on the shore of Lake Huron, 

 and from the corresponding section at Thedford across the lake on the Canadian 

 shore, arranged by A. Grabau to show the relationships of the various mutations. 

 In the scale of strata at the right 8J4 mm. ec^uals 100 feet depth. 



changes (saltations) or discontinuously, and without any defi- 

 nite direction or adaptive trend {Mutationsrichtung) . The 

 essential feature of de Vries's observations, in contrast to 

 Waagen's, is that of discontinuous saltations in directions that 

 are entirely fortuitous — that is, either in an adaptive or in- 

 adaptive direction, the direction to be subsequently deter- 

 mined by selection — a theoretic principle agreeing closely with 

 that of Darwin. 



