THE MUTATION THEORY 



286. The word mutation is one of the loose, ill-defined terms 

 so common in biology and the bane of it. Thus, to Mr Punnett 

 it implies a variation, large or small, which is stable because the 

 inheritance of it is Mendelian. To Professor de Vries it implies 

 a ' sport/ 1 a change " of wide amplitude," 2 the inheritance of which 



rendered to this branch of philosophy is the demonstration of the fact that the 

 mutation when once it has arisen is not likely to be swamped by interbreeding 

 with the normal form, provided that it is not injurious to the species. We now 

 recognize discontinuity in inheritance as well as in variation. The new character 

 which arises as a mutation has its representative in the gamete. Once it has 

 arisen selection alone can eliminate it. Mendel's discovery then has led us to 

 materially alter our ideas of the evolutionary process. The small fluctuating 

 variations are not the materials on which selection works. Such fluctuations are 

 often due to conditions of the environment, to nutrition, correlation of organs, 

 and the like. There is no indisputable evidence that they can be worked up and 

 fixed as a specific character " (Mr R. C. Punnett, Mendelism, ed. ii., pp. 72-3). 

 " Modification of characters by selection, when sharply alternative conditions (i.e. 

 mutations) are not present in the stock, is an exceedingly difficult and slow process, 

 and its results of questionable permanency. Even in so-called ' improved ' 

 breeds, which are supposed to have been produced by this process, it is more 

 probable that the result obtained represents the summation of a series of mutations 

 rather than that of ordinary fluctuating variations. For mutations are per- 

 manent ; variations are transitory. A moment's reflection will indicate the 

 probable reason. Variations which are distributed symmetrically about a modal 

 condition, so as to produce when graphically expressed a frequency of error curve, 

 represent the result of a number of causes acting independently 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 upon 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 ap- 

 parently is cross breeding " (Professor M. E. Castle, " The Mutation Theory of 

 Organic Evolution, from the Standpoint of Animal Breeding." Science, April 7, 

 1905). " Lop-eared rabbits have ears two or three times as long and as wide 

 as those of ordinary rabbits. A cross between lop-eared rabbits and ordinary 

 ones produces offspring with ears of intermediate size, which sometimes stand erect 

 and sometimes lop. The ear characters which were so distinct in the parents have 

 in this case lost their identity in the offspring, and apparently cannot be recovered 

 again in the original condition, for the offspring transmit to their young the blended 

 character rather than the extreme conditions found in their respective parents " 

 (Castle, " Recent Discoveries in Heredity" Popular Science Monthly, pp. 194-5). 

 Professor Castle refutes himself in a very interesting way. Probably no one will 

 deny that the lop-eared condition has an " internal origin in the hereditary 

 substance itself." Certainly it is quite permanent in the race and has no appear- 

 ance of depending on " varying conditions of food-supply, temperature, density, 

 moisture, light," etc. But it blends with the ordinary ear. Therefore it cannot 

 have resulted from a mutation the inheritance of which was Mendelian ; for 

 if it blends now, it must have blended formerly. Therefore it must have resulted 

 from the selection of fluctuations the inheritance of which was blended. 



1 Species and Varieties, p. 191. 



*Op. cit., p. 715. 



