i] Purity of Type 17 



or factors in their germ-cells, the paradox at once follows 

 that an organism may be pure-bred in respect of a given 

 character though its parents were cross-bred in the same 

 respect. Purity depends on the meeting of two gametes 

 bearing similar factors, and when two similarly-constituted 

 gametes do thus meet in fertilisation, the product of their 

 union is pure. The belief, so long prevalent, that purity 

 of type depends essentially on continued selection is thus 

 shown to have no physiological foundation. 



Similarly it is evident that an individual may be pure in 

 respect of one character and cross-bred or impure in respect 

 of others. 



As a consequence of the application of Mendel's prin- 

 ciples, that vast medley of seemingly capricious facts which 

 have been recorded as to heredity and variation is rapidly 

 being shaped into an orderly and consistent whole. A new 

 world of intricate order previously undreamt of is disclosed. 

 We are thus endowed with an instrument of peculiar 

 range and precision, and we reach to certainty in problems 

 of physiology which we might have supposed destined to 

 continue for ages inscrutable. 



After such a discovery it is obvious that old ideas must 

 be revised. Systematists debating the limits of " specific 

 rank " or the range of variability, morphologists seeking to 

 reconstruct phylogenetic history, physiologists unravelling 

 the interaction of bodily functions, cytologists attempting 

 to interpret the processes of cell-division each of these 

 classes of naturalists must now examine the current con- 

 ceptions of his study in the light of the new knowledge. 

 The practical breeder of animals or plants, basing his 

 methods on a determination of the Mendelian units and 

 their properties, will in many of his operations be able to 

 proceed with confidence and rapidity. Lastly, those who 

 as evolutionists or sociologists are striving for wider views 

 of the past or of the future of living things may by the use 

 of Mendelian analysis attain to a new and as yet limitless 

 horizon. 



B. 



