380 MUTATION 



of which the new characters can be transmitted to the offspring. 

 So far all attempts to demonstrate satisfactorily the inheritance 

 of such acquired characters have proved a failure. It is indeed 

 difficult to conceive of any plausible means by which characters, 

 acquired by the plant in the course of its lifetime, could affect and 

 modify the hereditary mechanism which must obviously be con- 

 tained within the fusing gametes. On the other hand, the marked 

 adaptations of plants in the wild state to their normal environ- 

 ment, features often retained when the organism is transferred 

 to another habitat, naturally suggest the possibility of the 

 unstable acquired characters becoming ultimately stabilised. 

 This view has led to the conception of the origin of new species, 

 etc., through cumulative selection and hereditary transmission 

 of favourable variations (or mutations, cf. below), tending 

 towards more complete harmony between the plant and its 

 environment. Another point, upon which there is much differ- 

 ence of opinion, is the actual influence of the environment in 

 moulding the structure of a plant. Whilst some consider that 

 adaptation to the environment is due to selection, others believe 

 in a direct response to changed surroundings. 



Seeing that the individual variations above considered are 

 all encompassed within the apparently fixed range of the species 

 (cf . p. 377) , they could scarcely seem to have led to the evolution 

 of new forms. But in carefully selected and self-fertilised 

 cultures of the higher plants, and even in pure cultures of lower 

 organisms, it has been found that slight or pronounced departures 

 from the mode occasionally arise which breed true to their new 

 characters from the very first, i.e. these are hereditarily trans- 

 mitted. Such mutations, which are sometimes far more pronounced 

 than the individual variations, and consequently obvious even 

 to superficial observation (Fig. 222), may well be responsible 

 for the origin of new species. As to the causes of mutation 

 we are, however, in complete ignorance, though it is tempting 

 to assume that the external environment is the stimulus that 

 brings about the internal change. If this could be experi- 

 mentally proved, many of the divergent views at present held 

 could be harmonised. 



A familiar instance of a mutation is afforded by the Irish 

 Yew (Taxus baccata, var. fastigiata), which differs from the 



