ENVIRONMENT AND EVOLUTION KROPOTKIN. 423 



mutation must have " not only an inner cause, but also an exterior 

 cause," and that the high variability of the Oenotheras must be u to 

 some extent a consequence of the special conditions of the soil,'" 1 

 has thus given a hard blow to the idea of a fundamental distinction 

 between " mutations " and ordinary variation. Both are inherited, 

 the difference being only one of degree in the modifying cause. 



It may be added that Erwin Baur, Avho also has carefully studied 

 the subject, comes to a similar conclusion in his Introduction to the 

 Experimental Theory of Heredity. As a rule (he writes) muta- 

 tions are rare (one in a thousand individuals, or less) ; and " what 

 are their causes in most cases we don't know." Only lately experi- 

 ments were made showing that mutations (i. e., inheritable variations) 

 can be provoked by exterior influences, depending on our will. Such 

 are the experiments on the Colorado beetle made by Tower, who 

 used high temperatures, dryness of the air, and low atmospheric 

 pressure, those of Blaringhem, who provoked inherited variations 

 by mutilations of plants, and MacDougal, who acted directly on 

 the reproductive cells. 2 



Finally we learn from another most careful and gifted experi- 

 menter. Professor Klebs, that those characters of a plant which 

 belong to the most constant ones under the ordinary conditions of 

 culture can become most variable under properly chosen conditions; 

 and that both the so-called continuous and the discontinuous varia- 

 tions (the mutations) can be obtained in the same individual, accord- 

 ing to the external conditions into which it is placed. 3 



The consensus of opinion is thus against attributing to mutations 

 an origin quite different from the origin of habitus variations. But 

 once it is so, we have in the so-called " mutations " another vast cate- 

 gory of characters "acquired" under the influence of a changed nutri- 

 tion in a new environment, and inherited. 4 And these two vast cate- 



1 De Vries, Gruppenweiso Artbildung, pp. 342—343 ; also Species and Varieties : their 

 Origin by Mutations, Lectures before the University of California, edited by D. T. 

 MacDougal, Chicago, 1906, p. 451. 



2 Erwin Baur, Einfiihrung in die exporimentelle Vererbungslehre, Berlin, 1911, pp. 

 202-204. In a recently published work by R. Ruggles Gates, The Mutation Factor in 

 Evolution, with particular reference to Oenothera (London, 1915), we have an important 

 contribution to this subject. Its chief interest is in the researches made by the author 

 to discover the changes which take place in the germ cells when an inherited variation 

 takes place in the extremely variable complexus of species and varieties represented by 

 the Oenothera. These researches have not yet brought the author to a definite conclu- 

 sion as to the causes of mutations (p. 321) ; but they open an interesting branch of 

 Investigations in the great question of heredity. 



3 "Studien fiber Variation," in Roux's Archiv, vol. xxiv, pp. 29—113; review in Annee 

 biologique, xiv, p. 357. 



4 With all the respect I have for the always most accurate work of Prof. J. Arthur 

 Thomson, I confess that, whatever his other reasons in favor of discontinuous variation 

 may be, the facts he mentions in Heredity (London, 1908, pp. 86-S9) hardly prove that 

 " variation leads by leaps and bounds." The very words with which Prof. Thomson 

 accompanies, with his habitual fairness, each of the examples he mentions,- suggest that 

 there is no reason to affirm and some reason to doubt that the new characters appeared 

 suddenly. About the wonder horse with an extremely long mane we are told that " the 

 parents and grandparents had unusually long hair"; about the Shirley poppy, that the 

 "single discontinuous variation" from which it was obtained "may have occurred often 

 before Mr. Wilks saved it from elimination " ; but no reason is given to suggest that it 

 was a " sudden " variation ; the same applies to the star primrose, the moth Amphirlasys, 

 and the medusoid Pseudoclitia pentata, which is said to be " remarkably variable." 



