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ducing poison, pigment, or spores, returns. Attention has 

 hitherto been confined to the so-called fluctuating variability, 

 and it must be remembered that, although in a series of such 

 variations, the two extreme forms may be widely dissimilar, 

 these are always linked together by a number of intermediate 

 forms which differ very slightly from each other; there is 

 no gap in the chain and no sudden and considerable variation, 

 only a number of very slight differences. We have also seen, 

 that such variations are inconstant, although in some cases 

 at least, under suitable conditions, they can be gradually fixed 

 and rendered hereditary. 



In contrast with such variations are those known as spon- Spontaneous 

 taneous variations, sports, or mutations. Our knowledge of Variations, 

 these and of their mode of occurrence in nature depends 

 chiefly on the experiments carried out by Professor Hugo De 

 Vries. He has demonstrated that plants of the species Oenothera 

 Lamarckiana in addition to producing normal offspring bearing 

 the characteristic marks of their parents, produced also con- 

 siderable numbers of individuals which possessed definite and 

 appreciable characters not seen in their parents. Such indivi- 

 duals are termed mutants ; they suddenly come into existence, 

 there is no progressive change, and no intermediate forms are 

 found linking them with the species from which they spring, 

 they are due in fact to a sudden variation, i.e. a mutation. A 

 mutant, when isolated and fertilised with its own pollen, is 

 found to be constant from the time of its origin and to transmit 

 its essential characters truly to its offspring. This constancy 

 sharply distinguishes mutations from the inconstant fluctuating 

 variations. The species mentioned was found to produce several 

 different types of mutants, differing in various ways from the 

 original form, several individuals of each type usually appeared 

 simultaneously, while each type was liable to be reproduced at 

 intervals by the parent species. The causes of mutations are at 

 present unknown. The great majority of species in nature 

 are fixed and constant, but from the case of this Oenothera 

 it appears possible that all species at some period of their 

 existence throw off mutants in considerable numbers. Such 

 a mutating period probably forms only a small part of the 

 total life of a species. Such a period moreover does not neces- 

 sarily involve the death of the parent species which, as seen 

 in this Oenothera, in addition to producing mutants, ma}' 

 continue to transmit its specific characters truly to the majority 

 of its offspring. Mutations are not entirely confined to such 

 mutating periods; occasional mutants may arise at any time, 



