154 Report of the State Geologist. 



Spencer,^ by Semper,t by CopeJ and the American school of 

 Neo-Lamarckians. Spencer insists on the effects of use and 

 disuse and shows that very small variations in the force of an 

 organ can be of no service to the individual nor thus preserved by 

 natural selection. These objections appear to have much embar- 

 rassed Wallace, who replies by the enunciation of a new law due to 

 Galton — the law of the return to the Tnecm : When any part has 

 been increased or diminished by selection, there is among the 

 progeny astrong tendency to return to the mean condition when- 

 ever the influence of selection ceases to act. The degeneration 

 of the atrophied parts might also Tdc explained by a utilitarian 

 purpose; an organ too weak becomes a source of danger and 

 should disappear by selection. 



The whole question becomes reduced to two terms which are 

 easy to define. 



1. Are there really individual modifications which are due 

 directly to variations of the medium ? 



2. If the modifications in question are produced, can they be 

 transmitted by heredity ? 



Influence of the medium. — The affirmative answer to the first 

 question has been given in particular by Semper, who supports 

 it by numerous examples drawn from the Mollusks. The recent 

 experiments made on plants, particularly by the Botanical School 

 of France, show in the structure of plants important and strictly 

 determinate variations as conspicuous in the higher types as- 

 in lower forms, like the Mushrooms. Facts of this kind form 

 the basis for the methods everywhere applied for the transforma- 

 tions of pathogenic microbes in vaccinations. Accurate experi- 

 ments have been made on beings of much higher organization. 

 Whitfield, Semper, Locard, Clessin, Dall, Baudon, etc., have shown 

 that important variations were effected among Mollusks by 

 changes in the dimensions of the medium, in its agitation, in its 

 pressure. § 



These observations are not very easily explicable by the 

 theory of Weismann, which Wallace has accepted, of the non- 



* H. Spencer, Factors of Organic Evolution. 1886. 



+ Semper, The Natural Conditions of Existence as they Affect Animal Life. London, 1883. 

 i Cope, The Factors of Organic Evolution; The Natural Conditions of Existence as they Affect 

 Animal Life. London, 1883. The Origin of the Fittest. 1887. 

 § See Locard, LHnfluence du milieu sur le dSveloppement des 3Iollusques, 1892. 



