The Principles of Palaeontology. 155 



heredity of acquired characteristics^ Still it is certain that 

 variations produced directly and artificially are not generally so 

 strongly fixed that the modified type may not return to the primi- 

 tive type by a return to the first conditions. This notably happened 

 in the famous experiments of Schmankewitz on Artemia salina.\ 

 This Phyllopod Crustacean normally lives in brackish waters, 

 but being raised in waters more and more fresh, it evolved gradu- 

 ally, and at the end of some generations was transformed into a 

 very different form which had been described under the name of 

 BranchijpxLS stagnalis^ and which lived normally in fresh water. 

 On the contrary, by augmenting the saltnessof the water, Artemia 

 salina can be transformed into A. Milhauseni^ a species which 

 habitually lives in marine waters. But in the case under con- 

 sideration it is to be seen that, on the one hand, the variation is 

 not sufiiciently fixed to prevent the return to the primitive type 

 (whichever of the three species that may be) ; whilst, on the 

 other hand, the acquired characteristics are highly hereditary, 

 since in a given medium each of the three forms respectively per- 

 petuates itself with a persistency suflBcient to form a veritable 

 species. 



The direct or indirect influence of the medium on variation is 

 moreover an indubitable fact, but it remains to be determined 

 whether the variations thus transmitted are acquired by exercise 

 or disuse, or whether they are spontaneous variations of the ger- 

 minative plasma, accumulated through natural selection. 



Experimentation alone can furnish a conclusive solution of this 

 problem, which at the present day engages the attention of so 

 many naturalists. Palaeontologists, moreover, have entered into 

 the discussion, and have brought forward arguments more or less 

 theoretic, drawn from the study of fossils. 



American Neo-Lamarckism. — The theories of Cope:}: and of 

 Hyatt are enveloped in some metaphysical obscurities which 

 struck Darwin himself.§ The most important points are, first, 

 the acceptation of the influence of the medium ; and, next, the 



♦ On this theory (theory of the continuity of the^ germ-plasma) and its consequences see 

 Weismann, Essay on Heredity and ^''atural Selection; Ball, Are the effects of use and disuse hereditary? 

 Numerous articles by Vines, Turner, Weismann, Osborn, Mivart, Ryder, Lankester, etc, in. Nature 

 and American Naturalist. 1889, 1890, 1891. 



fSchm&nkewitz, Zeitschf. IV. Zool. 1877. 



* Cope, Origin of the Fittest. Essays on Evolution. New York, 1887. 

 S Life and Corresiiondence of Darwin. 



