CHAPTER VIII 



VARIATION AND HEREDITY l 

 SECTION 55. The Internal Conditions for Hereditary Variation. 



EXISTING species are so stabilized that they retain their essential 

 characteristics even under unusual cultural conditions, while the offspring 

 repeat the life-history of the parents when returned to the original conditions. 

 There are, however, exceptions to this rule, for alterations may take place 

 in a particular plant which are handed on to succeeding generations, and 

 may become a fixed hereditary property whether morphological or metabolic 

 in character. The variations observed by us are usually of relatively trifling 

 importance, and do not go so far as to lead to the appearance of an entirely 

 distinct species. Nevertheless the experimental study of variation affords 

 a key to the gradual changes which have peopled the globe with the varied 

 forms of life that now exist or have once existed. We are, however, only 

 concerned with the conditions which favour variation and maintain it 

 when produced, as well as those related facts which have a physiological 

 bearing. 



An hereditary variation must arise from some internal change in the 

 protoplast which is transferred by the germinal cells to the offspring, for 

 without these had undergone some permanent change the development 

 would under similar conditions follow the same course as that of the parents. 

 An alteration in any one of the component parts of the protoplast would 

 suffice to produce a recurrent divergence in the ontogeny, but the result 

 affords no evidence as to whether the seat of the alteration lies in the nucleus 

 or in the cytoplasm. Nor need the germinal cells undergo any visible 

 alteration. Indeed the seat and character of the modification in the germinal 

 cells has never been even approximately determined, except in cases where 

 the variation is symbiogenic, that is, produced by the entry and admixture 

 of living material. 



The production of hybrids shows that close union may occur between 

 dissimilar living substances to produce a new form 2 , and possibly hybridiza- 



1 In a recent work by de Vries (Die Mutationstheorie, 1901) the subject is discussed in detail. 

 De Vries uses the term ' variability ' in a more restricted sense, and distinguishes a sudden 

 variation as a mutation. 



2 Most hybrids undergo the progressive differentiation towards the parents observed by Mendel 

 in successive generations of fertile hybrids. Cf. Bot. Ztg., 1900, pp. 231, 304; also Correns, Bot. 



