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Such a view also satisfactorily accounts for the fact 

 that everywhere in nature we find animals and 

 plants exhibiting the most beautiful adaptations to 

 the conditions of their environment, by means of 

 which they are favoured in the struggle for 

 existence. 



Opinions, however, differ as to which type of variation has 

 played the most important part in the history of species. The 

 variations which are believed to be most important from this 

 point of view may be divided into three main classes as 

 under : 



(1) Variations due to the crossing of different 



forms, i.e. to hybridisation. 



(2) Fluctuating variations. 



(3) Mutations. 



It may also be premised that, in order to form the beginning 

 of a new species, a plant must be able to maintain itself under 

 the conditions of existence to which it is exposed in nature, it 

 must be able to transmit its essential characters truly to its off- 

 spring, it must occur in considerable numbers and be sufficiently 

 fertile to produce numerous offspring. Such a plant may possibly 

 result from hybridisation alone. As regards fluctuating vari- 

 ability, experience with bacteria indicates that, if the conditions, 

 to the stimulating action of which the variation must be 

 ascribed, are kept constant, in some cases, at least, a new and 

 constant form may be gradually evolved which may also be 

 able to establish itself as a new species. In the case of plants 

 reproducing sexually, such a progressive change would be 

 aided by natural selection, for all individuals not exhibiting the 

 variation, which, under the circumstances, is favourable to 

 their existence, would perish, while only those possessing the 

 variation in a high degree would remain to cross and produce 

 offspring. We have seen also that new, fertile, and constant, 

 forms in considerable numbers may arise by mutation, and 

 those which are able to survive in the struggle for existence 

 may form the beginning of new species. Just as the small 

 fluctuating variations may be gradually accumulated and 

 increased in the same direction by natural selection, so 

 may be also mutations, for, providing the conditions remain 

 constant, of all the mutants produced during successive 

 mutating periods only those mutants which vary in the 

 same direction will be able to survive, and thus evolution 

 will continue along the same lines, just as with fluctuating 



