172 DR. H. B, FAXTHAM 



factors or unit characters, with regard to which there 

 is a complete segreoation among the germ cells, each of 

 which bears one only of each pair of contrasted charac- 

 ters. A table of some of the dominant and recessive 

 characters of plants and animals was shown, — for 

 example, tall and dwarf stems in peas, yellow and green 

 cotyledons in peas, round and wrinkled seeds in peas, 

 susceptibility and immunity to rust in wheat, rose comb 

 and single comb in fowls, eye-colour in man, certain 

 diseases in man (such as brachydactyly, night-blindness, 

 colour-blindness, haemoj)hilia, and presenile cataract). 

 Bateson. Punnett and Bitfen have done much work, on the 

 elucidation and practical application of Mendelian prin- 

 ciples in England. 



Xew varieties nmy arise by crossing, and some species 

 may have arisen in that manner, according to Lotsy. 



A mode of origin of species, derived from his study of 

 variations, was set forth by Professor Hugo de Vries, of 

 Amsterdam, in his work entitled " The Mutation Theory." 

 published in 1900. His conclusions were based upon the 

 study of plants, the classical example being his work on 

 the evening primrose, Oenothoa lamarcJciana. From his 

 observations, he concluded that species arise from one 

 another by changes of considerable magnitude, accom- 

 plished by discontinuous leaps and bounds. De Vries' 

 own statement was that ^^ tlie new species appears all at 

 once; it originates from the parent species without any 

 visible preparalioi^ and witliout any obvious series of 

 transitional forms." Further, these s])ecies are constant 

 from the first. Tlie discontinucms variations or muta- 

 ti(ms of de Vries ai-e, in the main, the outward manifesta- 

 tions of the presence or absence of the corresi)onding 

 Mendelian factors. Indeed, ^Mendel's experiments de 

 monstrated the existence of discontinuous variations. 



A Honuonc hypothesis of heredity has been set fortli 

 by Cunningham and others. A hormone is an internal 

 secretion or chemical messenger produced by some organ 



