Lefevre — The Advance of Zoology in the Nineteenth Century. 93 



However slight a given variation might be above the mean, in 

 times of stress the survivors would contain a relatively larger 

 number of individuals possessing that advantage, and this 

 being repeated at subsequent generations, natural selection 

 would establish adaptations by thus gradually raising the 

 general average. The advance would therefore be, not by 

 isolated spurts of individual variations, although it is possible 

 that this might take place in certain cases, but an advance of 

 the species as a whole. And furthermore, as it is largely 

 those individuals falling below the mean which are extermi- 

 nated, the survivors would have to mate with each other, and 

 there would be no opportunity for the new character to be 

 eliminated by cross-breading. 



The necessity of variations occurring in definite, beneficial 

 lines, as opposed to indiscriminate variation, in order to pro- 

 duce adaptations, that is, the necessity of their being deter- 

 minate, has for a long time appealed with force to the minds 

 of some. If variations are determinate, if there is some 

 underlying cause which calls forth the variation when needed 

 and directs its development, then, it is argued, Natural Se- 

 lection would not be the cause of evolution, and the real 

 problem of evolution would be the discovery of the cause of 

 the determinate variation. There has arisen a school of 

 biologists who, working from this standpoint, have attempted 

 to identify this cause with the old Lamarckian factors but 

 with modifications necessitated by the advance of zoological 

 knowledge. The all-sufficiency of Natural Selection as the 

 cause of evolution, is denied by these Neo-Lamarckians who 

 maintain that use and disuse and the action of the environ- 

 ment produce and determine variations, directing them along 

 beneficial lines ; and moreover, that the effects of these factors 

 upon the individual are transmitted. They assign to Natural 

 Selection only a secondary role in contributing to the estab- 

 lishment and elaboration of variations after they have once 

 been produced by use or disuse or by the action of the envi- 

 ronment and brought into existence when needed. During 

 the past twenty or twenty-five years the contention over 

 Natural Selection versus the inheritance of '' acquired char- 

 acters " has proceeded with considerable earnestness, and 



