362 APPENDIX B 



diverged by gradual cumulation of minute differences in size, or 

 by a single step ? These alternative views are known respectively 

 as the selection theory and the mutation theory." 1 Other writers, 

 however, imply that mutations are not necessarily distinguished 

 by magnitude. Thus Mr. Punnett writes : " The difficulty of 

 distinguishing between the two is very great. The simultaneous 

 existence of small mutations and large fluctuations leads to the 

 disguising of the former by the latter . . . . it is by the 

 selection of mutations, often very small, that the gardener 

 improves his varieties." 2 De Vries limits the term "mutation" to 

 discontinuous progressive variations, and strenuously denies that 

 mutations are subject to Mendel's law. 3 Most mutationists, how- 

 ever, seem to apply the term to regressions as well as progressions, 4 

 and some of them insist that a mutation is distinguishable from a 

 variation only by the fact that inheritance as regards the former 

 is Mendelian, whereas it is blended as regards the latter. Thus 

 Mr. Punnett writes : " The magnitude of the mutation may be 

 great and striking, or it may be comparatively small. But what- 

 ever its size its inheritance would seem to be according to the law of 

 gametic segregation." 5 Occasionally language is used which implies 

 that the writers regard all variations as (Mendelian) mutations, 

 the term "variation" being then applied by them to the characters 

 which we have termed "acquirements." 6 



Here, then, we have the rival theories of evolution and heredity 

 the selection theory, which supposes that evolution depends on 



1 The Mutation Theory of Organic Evolution from the standpoint of Animal 

 Breeding, by Prof. W. E. Castle, Science, Apiil 7th, pp. 521-525. 



2 Mendelism, pp. 51-2-3. 



3 Species and Varieties : Their Origin by Mutation. Lectures IX and X. 



4 See, for example, Mendelism, p." 49. "It is probable that the dwarf 

 pea arose as a mutation from the tall." 



5 Mendelism, p. 47. See also The Mutation Theory of Organic Evolution 

 from the standpoint of Animal Breeding, by Prof. W. E. Castle, p. 6. 

 According to Mr. Bateson and Miss Saunders, "such discontinuity will in 

 fact depend not on the blending or non-blending of the characters, as hitherto 

 generally assumed, but on the permanent discontinuity or purity of the 

 unfertilized germ-cells." Eeport, I, to Evolution Committee of Royal Society, 

 p. 130. 



6 ' ' Mendel's discovery then has led us to materially alter our ideas of the 

 evolutionary process. The small fluctuating variations are not the material 

 on which selection works. Such fluctuations are often due to conditions of 

 the environment, to nutrition, correlation of organs, and the like. " (Punnett, 

 Mendelism, p. 52 ) " Variations which are distributed symmetrically about 

 a modal condition, so as to produce, when graphically expressed, a fre- 

 quency of error curve, represent the result of a number of causes acting in- 

 dependently of each other. These causes are principally external, consisting 

 in varying conditions of food-supply, temperature, density, moisture, light, 

 etc. These conditions alter from generation to generation, and so do effects 

 dependent on them. Mutations, on the other hand, have an internal origin 

 in the hereditary ' substance ' itself. They are relatively independent of 

 the environment, being affected only by such causes as affect the nature of 

 the hereditary substance itself, one of which is apparently cross-breeding." 

 (Castle, Op. cit., p. 8.) 



