IRISH GARDENING. 



^^S 



Mendelism. 



By Professor JAMES WILSON, M.A., B.Sc. 



INCE the days of 

 Darwin there have 

 been no more il- 

 knnuiative workers 

 for breeders of 

 plants and animals 

 than De Vries and 

 Mendel. It may be 

 heresy to say so, 

 but, for such 

 breeders, De Vries 

 and Mendel have 

 been far more il- 

 luminative than 

 Darwin himself. Darwin showed us that 

 plants and animals vary, but, as he thought, 

 slowly and constantly : at any rate that racial 

 changes are brought about by slow and constant 

 variations. De Vries has shown us that 

 changes come suddenly, and Mendel has fur- 

 nished the explanation, or, at any rate, some 

 part of the explanation. Listen to a very 

 beautiful description of De Vries's work from 

 Professor Arthur Thomson's recently published 

 "Heredity": — 



" hi 1886 De Vries began hunting; about around 

 Amsterdam for a plant which Would show hints of being 

 in what we may call a changeful mood. He tried over 

 a hundred species, bringing them under cultivation, but 

 almost all were disappointingly conservative. It seemed 

 as if inost of the species around Amsterdam were in a non- 

 mutable state. It is possible, as Weismann suggested 

 in one of his first evolutionary essays (1872), that in the 

 life of species, periods of constancy alternate with 

 periods of changefulness. The human historian has 

 often made a similar remark. 



" In the course of his wanderings around Amsterdam, 

 De Vries came across a deserted potato-field at 

 Hilversum — a field of treasure for him. For there he 

 found his long-looked-for mutable plant, an evening 

 primrose ( GLnoihera lamarkiana ). Like its nearest re- 

 latives, (Enothera biennis and CEnothera tnuricata, which 

 it excels in size and beauty of flowers, it probablj' came 

 from America, where it is a native. It had probably 

 'escaped' at Hilversum about 1875, and in the 

 following ten years it had spread in hundreds over the 

 field. It had been extremely prolific in its freedom ; but 

 that was not its chief interest. 



"Its chief interest was its changefulness. It had, so 

 to speak, frolicked in its freedom. Almost all its 

 organisms were varying — as if swayed by a restless tide 

 of life. It showed minute fluctuations Irom generation 

 to generation ; it showed .ex.traordinary freaks like 

 fasciation and pitcher-forming ; it showed hesitancy as 

 to how long it meant to live, for while the majority were 

 biennial, many were annual, and a few were triennial ; 

 best of all, it showed what can hardly be otherwise 

 described than as new species in the making. 



" It is possible that the prolific multiplication in a new 

 environment may have had something to do with the 

 awakening of the impulsive mutability. 



" In 1887, a year after his discovery of the potato-field, 

 De Vries found two well-defined new forms — a short- 

 styled O. brevisfylis and a beautiful smooth-leaved 

 O. laevifolia - distinguishable from the parent in many 



details. He hailed these as two new ' elementary 

 species,' and he applied one of the crucial tests of 

 specific or sub-specific rank : Did they breed true? He 

 found that this was so ; from their self-fertilised seeds 

 similar forms arose. Neither of the two new forms was 

 represented in the herbaria at Leyden, Paris, or Kew ; 

 neither had been described in the literature oiOnagnicece. 

 They seemed to be distinctly new. It is interesting to 

 note that in 1887 there were few examples of these two 

 new elementary species, and that each occurred on a 

 single plot on the field. The impression conveyed was 

 that each had arisen — by a sudden mutation — from the 

 seed of an individual parent. 



"The next chapter in the famous investigation began 

 with a transference of samples of the new forms and the 

 parent stock — partly as plants and partly as seeds — 

 from the potato-field at Hilversum to the botanic garden 

 at Amsterdam. 



" The three stocks gave rise under cultivation to 

 many thousands of individuals, which bred true along 

 certain lines, and yet gave rise to other new forms. 

 In short, De Vries had found a plant in the process of 

 evolution. 



"The predisposition to mutability — which remains a 

 mystery — was present. De Vries gave it scope, and 

 like the primeval gardener he had the pleasure of 

 giving names to a crop of new creations which emerged 

 before him. From each of these three samples there 

 arose distinctive groups— which if they had been found 

 in nature would have been reckoned as distinct species 

 of evening primrose. But the most interesting feature 

 was the apparent abruptness in the origin of the new 

 forms. They seemed to rise by leaps and bounds, by 

 organic jerks ; they illustrated what De Vries has 

 called ' mutation.'" 



Mendelism comes in now to explain the 

 advent not of these new forms perhaps, because 

 I do not know enough about the evening prim- 

 rose to say, but certainly of other new forms of 

 plants and animals with which gardeners and 

 farmers are well acquainted. A black breed of 

 fowl is bred with a white breed, and the young 

 are blue. When these blue hybrids are bred 

 together only a half of their young again are 

 blue : one half the remainder being black like 

 one of their grandparents and the other half 

 white like the other. 



Mendel formulated a theory to explain this. 

 He conceived the idea that every plant and 

 animal must carry from its very beginning a 

 lot of somethings which determine its future 

 character : one to determine its colour, another 

 its size, another the shape of one part, another 

 the shape of another, and so on. He further con- 

 ceived the idea that these determinants must be, 

 as it were, twins : that each must have two 

 component parts. This theory explains many 

 phenomena in variation and heredity. 



Let us take a simple case first, the case of 

 the fowl mentioned above. The black fowl 

 carried a colour determinant for blackness, the 

 white fowl another for whiteness. Let us 

 represent the black determinant by two small 

 black circles and the white by two white ones, 

 thus :— #0 



• O 

 The black determinant being carried by one 

 parent and the white by another, the young 



