214 MENDELISM 



of mice. The simplest typical ratio thus produced is 

 13 : 3 : i. 



4. The last type of latency described by Dr. Shull 

 is of a totally different kind. It is represented by the 

 phenomenon to which the term ' latency ' was origin- 

 ally applied by de Vries. This latency consists in the 

 disappearance of certain characters under the influence 

 of poor nutrition, or other changes of conditions. 

 Examples of this phenomenon described in the chapter 

 on Mutation were the submerged and floating leaves 

 of the water-ranunculus, and the red and white colour 

 of the flowers of a species of primula. 



We may now proceed to pass in rapid review a 

 selection of the more remarkable instances of Mendel ian 

 inheritance which have been so far demonstrated. 



The ease with which characteristics of colour can be 

 distinguished and defined has naturally led to a good 

 deal of attention being paid to the phenomena of their 

 inheritance. In this way many cases of simple domi- 

 nance have been discovered in plants and in animals, 

 as well as several examples of reversion in F v followed 

 in both cases by a Mendelian segregation of characters. 



Thus the colours of many flowers afford perfectly 

 simple phenomena, whilst other cases, like the sweet- 

 peas and the closely similar case of stocks studied by 

 Miss Saunders, have required long and arduous ex- 

 periment for their elucidation. No case of this kind 

 hitherto examined has been definitely proved to be 

 non-Mendelian. 



Colour characters which follow MendeFs law have 

 been observed hi mice, rats, rabbits, guinea-pigs, 





