404 LAMARCK, HIS LIFE AND WORK 



ment and the habits of the animal were the efficient 

 cause of the change, and any explanation which ex- 

 cludes the direct action of such agencies is con- 

 fronted by the difficulty of an immense number of 

 the most striking coincidences. ... So far as 

 I can see, the theory of determinate variations and 

 of use-inheritance is not antagonistic but supple- 

 mentary to natural selection, the latter theory at- 

 tempting no explanation of the causes of variation. 

 Nor is it pretended for a moment that use and disuse 

 are the sole or even the chief factors in variation." 



As early as 1868 the Lamarckian factor of isola- 

 tion, due to migration into new regions, was greatly 

 extended, and shown by Moritz Wagner* to be a 

 most important agent in the limitation and fixation 

 of varieties and species. 



" Darwin's work," he says, " neither satisfactorily 

 explains the external cause which gives the first im- 

 pulse to increased individual variability, and con- 

 sequently to natural selection, nor that condition 

 which, in connection with a certain advantage in the 

 struggle for life, renders the new characteristics indis- 

 pensable. The latter is, according to my conviction, 

 solely fulfilled by the voluntary or passive migration 

 of organisms and colonization, which depends in 

 a great measure upon the configuration of the coun- 

 try; so that only under favorable conditions would 

 the home of a new species be founded." 



* " Cber die Darwinische Theorie in Eesug auf die geographische 

 Verbreitung der Organismen." Sitzenb. der Akad. Munchen, 1868. 

 Translated by J- L. Laird under the title, The Darwinian Theory 

 and the Law of the Migration of Organisms . London, 1873. Also 

 Ueber den Einfluss der geographischen Isolirung and Colonierbildung 

 auf die morphologischen Verdnderungen der Organismen. MUnchen, 

 1870. 



