CORRESPONDENCE 453 



similarly exposed develops a deep continuous coat of pigment similar to 

 that of the land -newt. 



In the case of the mutants we have the loss of something from the constitu- 

 tion ; in the case of the wild species we have the functional regulation of some- 

 thing, which is a widely different matter. The supposed instances of agree- 

 ment between Mendelian mutation and specific characteristic will have to be 

 critically tested before any of them can be admitted to be sound. 



Mr. Huxley takes me to task because I have called Prof. Morgan's pupils 

 " factor-mad." He admits that they have put forward wild theories, but 

 says that their enthusiasm is pardonable, and that their theories " have not 

 been disproved." No, and I shall venture to predict that they will never be 

 disproved ; for I am confident that, as fresh facts turn up, which are in- 

 consistent with these theories in their present form, new imaginary and 

 undemonstrable genes will be invented to account for them, and of that 

 process there is no end. 



If the view which I have put forward in the preceding pages is sound, it 

 is hopeless to expect, by analysing the monstrosities of the mutants, to 

 arrive at conclusions as to how the chromatin is built up. On this subject I 

 should like to quote some wise and pregnant words of Prof. R. Harper, 

 which, as retiring President, he addressed to the American Botanical 

 Association (" The Structure of Protoplasm," Amer. Journ. Botany, 1917) : 



" The realisation of the weakness of Mendelism in relation to facts as to the 

 all-pervasiveness and interdependence of plant characters has led some of 

 the defenders of the theory to assert that each unit-factor may possibly 

 influence every part of the mature plant. The difficulty with all such 

 assumptions of hereditary units of whatever kind is more fundamental. 

 Take the case of the serrations of the leaf of the common nettle. Correns 

 told us, in 1903, that entire margins and serrate margins were due to a factor 

 in the germ-plasm, for serratures paired with one for entire margin and the 

 whole was made an example of dominance and segregation. It is to be noted 

 that Correns found teeth weakly developed in his recessives, and I am willing 

 to predict, on the basis of my studies on sugar and starch characters, and 

 aleurone colour in corn, that a whole series of intermediates between serrate 

 and entire can be found, and that a present-day student, instead of saying 

 that there is one factor for toothed margin, would say there are several, 

 perhaps twenty. If serrateness and entirety were found to be absolutely 

 hard-and-fast categories, there might be something to be said in favour of 

 assuming that each was represented by an equally hard-and-fast, definitely 

 limited section of a chromosome. But, if there are all degrees of variation 

 from entire to deeply serrate, the existence of a series of units in the germ- 

 plasm, one for each depth of serrateness, is not obviously suggested. The 

 series of fluctuating units has a unity to the human mind quite as natural 

 as any one of the particular grades of serrateness. This is evidently felt 

 vaguely by those who assume modifying factors, and factors of fluctuating 

 potency. To assume, however, that we have explained anything or in any way 

 contributed to clear up our knowledge of germ-plasm or heredity by saying that 

 the fluctuating behaviour of visible characters is explained by modifying 

 factors in the germ-plasm is a preformationism, which would have put even 

 Bonnet to the blush. 



"Perhaps the most obvious weakness of all those theories is that they 

 carry in them the vices of the old preformationism. They seem too much like 

 attempts to explain visible and familiar complexity by the assumption of a 

 parallel complexity in the germ-plasm, and this, in spite of the admitted 

 incommensurability of cell-organisation and metaphyte organisation. Driesch, 

 with all his tendency to mysticism, must be credited with having recognised 

 and made clear that the facts of nuclear and cell-division, and the resulting 

 perpetuation of the hereditary complex, made it impossible to assume a 



