.DO PRESIDENTIAL ADDRESS SECTION D. 



There is established a natural selection, i.e., a choice which per- 

 petuates the advantageous variations and eliminates the others. 



Among Neo-Lamarckians, A. Giard accorded to natural selec- 

 tion the value of a secondary factor. Among the Neo-Darwinians 

 we have Weismann, who believed in the continuity of the germ- 

 plasm, denied all value to Lamarckian factors, and saw in selection 

 the predominant factor in evolution. Such was approximately the 

 state of affairs at the end of the 19th century. In this, the 

 twentieth century, however, two kinds of investigation have de- 

 veloped, namely, the methodical study of variations and the 

 systematic study of heredity, especially of hybridisation. The 

 two kinds of investigation overlap. De Vries (1901) studying the 

 Evening Primrose [Oenothera laniarckiana) considered that he 

 found new species suddenly arise by saltations, and to these 

 sudden appearances he gave the name mutations. Further study 

 has led to the distinction of two kinds of variations, namely : 

 (1) small casual ones called fluctuations produced under the 

 influence of the environment but not hereditary, and (2) large dis- 

 continuous variations or mutations, not directly dependent upon 

 the environment but upon heredity. It may be added, however, 

 that some of the most recent investigations indicate that the dis- 

 tinction between these variations must not be too greatly 



■ emphasised. The important new branch of biology termed 

 genetics has arisen from these studies. 



In these important researches the reconsideration and exten- 

 sion of the valuable work of Abbot Gregor Mendel on hybridisa- 

 tion, announced in 1865 but overlooked until 1900, has formed a 

 basis. The fundamental principle of Mendel's results was that the 

 unit characters or factors contributed by two parents of a cross 

 separate, in the germ cells of the offspring without having had any 

 influence on each other. For example, Mendel himself worked on 

 crossing different varieties of the garden pea. He found that of 

 certain contrasted couples of parental characters or factors, such 

 as tallness and dwarf ness, which did not blend, one was dominant 



•over the other, which was latent or recessive. The first genera- 

 tion of hybrids was apparently all dominant, but it Avas subse- 

 quently found that they were really impure dominant. When 

 these hybrids were inbred it was found that one-quarter of them 

 reverted to the dominant type, one-quarter to the recessive type, 

 while one half reproduced hybrid features, and that these propor- 

 tions were maintained when the impure dominants were again 

 inbred. The inbred offspring of pure dominants and pure reces- 

 sives bred true. More than one pair of contrasted characters or 

 factors may differ in parents. In crossing, the two pairs of factors 

 segregate independently of each other, and the ratio 9:3:3:1 

 is characteristic of dihybrids when one member of each pair of 

 characters is dominant. Strict supporters of these principles con- 

 sider that Mendelian factors are unchangeable. 



Again, reference should be made to the work of Johannsen in 

 1903 and onwards on self-fertilising pure-bred plants like beans, 

 where there is a continuation of one and the same line, for tl'e 



