OTHER THEORIES OF SPECIES-FORMING. 39 



Protozoan exactly comparable, in the sense of strict continuity of 

 reproduction, to the germ-cell of a Metazoan, and yet failed to 

 note that a Protozoan can be influenced by environmental change. 



"Accordingly, an environmental change may be capable of induc- 

 ing change in the energies of the germ-plasm. The expression of 

 the energies of this substance, as viewed in temporal succession, 

 constitutes heredity. Therefore, the process of heredity may become 

 modified by a change in the environmental conditions of the germ- 

 plasm. And since we defined variation as a modification of heredity, 

 a variation could be produced by external influences acting upon 

 the germ-plasm, understanding by external influences influences of 

 the tissue-cells upon the germ-cells, or, in case the germ-cells are 

 not enclosed, influences of the non-living environment." 



In a recent exhaustive paper by Tower, W. L., on ''Evolution in 

 Chrysomelid beetles of the genus Leptinotarsa," Publication No. 48 

 of the Carnegie Institution of Washington, 1907, the author, although 

 on the whole a strong adherent of selection as the all-important 

 factor in species-forming, states specifically his belief, on the basis 

 of extensive observation and experiment, that external factors may 

 and do influence the germ-plasm to the extent of compelling it to 

 produce variations. These variations will not be photographic re- 

 productions of modifications of the soma, but they will be the vari- 

 ations which lie at the basis of species change. In other words, 

 Tower holds that variations are epigenetic in their origin, although 

 they are manifest as congenital differences. That is, acquired 

 characters in the usual sense of the term are not heritable, but all 

 variability is nevertheless due to the influence of environment. This 

 paper by Tower is a distinctly valuable contribution to our knowl- 

 edge of heredity and species change and it is based upon a large 

 amount of actual close observation and experiment. 



In a recent paper by Jonathan Wright, "The Origin and Heredity 

 of Matter," in the St. Louis Medical Review, 1906, something of the 

 same attitude is taken, although the author is much more inclined 

 to the belief in the general heritability of acquired characteristics. 

 This short paper is an admirable treatment, based on a full acquaint- 

 anceship with the modern literature on the subject, of the problem 

 of heredity. 



3 See exposition of a number of these theories in appendix of 

 chapter viii of this book. 



4 Haacke, W., "Grundriss der Entwicklungsmechanik," p. 289 

 ff., 1897- 



6 Plate, L., "U'ber die Bedeutung der Darwin'schen Selections- 

 prinzip," p. 218, 1903. 



6 The following account of the scientific aspects of Luther Bur- 



