584 BOTANY OF THE LIVING PLANT 



that, where it has come into competition with either 5. stricta or S. alterniflora, 

 the new species has gained the upper hand. 



Another example of polyploidy in Nature is provided by Biscutella laevigata, 

 a Crucifer native to Central Europe and Italy, which has some races with 18 

 chromosomes (diploid) and others with 36 (tetraploid) . The tetraploid races 

 occur over a wide and continuous geographical area, ranging from the Alps 

 to the Balkans. The diploid races, on the other hand, occupy several com- 

 paratively small and discontinuous areas in the valley-systems of the Rhine 

 and of some other large rivers. The explanation suggested is that the diploid 

 races are representatives of an ancient (inter-glacial or pre-glacial) type, while 

 the tetraploid races are post-glacial immigrants. A similar state of affairs has 

 been noted in the case of several North American species of Tradescantia. 



That polyploidy las played a considerable part in producing new species is 

 indicated by a more general line of evidence. The species of Angiosperms for 

 which the chromosome numbers are known (several thousands) belong to 

 about 500 genera representing a diversity of Families ; roughly 16 per cent, 

 of these genera are made up of species forming polyploid series. Thus in 

 various species of Chrysanthemum the somatic numbers are 18, 36, 54, 72 and 

 90, i.e. the basic number is 9, and the condition of the several species ranges 

 from diploid to decaploid. In Solarium the basic number is 12, and the series 

 ranges from 24 to 144. In Papaver, finally, two series can be distinguished 

 with 7 and 11 as their respective basic numbers. 



The general importance of polyploidy has been summarised as follows by 

 Crane and Lawrence : it increases the effective range of hybridisation ; it 

 combines the products of specific differentiation within a single (new) species ; 

 and it increases the potential range of species variation. There may also be an 

 increase in vegetative vigour, frost-resistance, and resistance to pathogens. 

 In this connection, it is significant that so many valuable cultivated plants, 

 for example Wheat and Oat, are polyploids. 



Genetics and Evolution. 



In the foregoing condensed and therefore necessarily somewhat 

 dogmatic account only a few of the more important aspects of genetics 

 have been considered. Among the interesting topics which have been 

 passed over as unsuitable for inclusion in an elementary discussion 

 are : the nature and inheritance of sex ; hybrid vigour ; the definition 

 of a " species " ; the origin and biological significance of dominance ; 

 the causes of mutation ; the nature of genes ; and the mechanism 

 of the action of genes upon developmental processes. The very 

 simple cases of Mendelian inheritance and of polyploidy which have 

 been considered in some detail may, nevertheless, provide a basis for 

 some general reflections upon the relation of genetical data to the 

 wider problem of evolution. 



The outstanding features of Mendelian phenomena are two : 

 (1) Inheritance is seen to he particulate, the characters of the parents 

 being transmitted individually to the offspring by genes situated at 



