292 PLANT-BREEDING 



called fluctuating deviations from the type. Useful qualities 

 are subjected to it as well as those practically useless marks, 

 which are usually studied merely on account of the valuable 

 indications they so often give for comparative science. 



Some of our most convincing arguments may be derived 

 from the study of teratological phenomena. Every one 

 who is collecting monstrosities, knows that they are, to a 

 high degree, dependent on the influence of the environment. 

 Some years are known to be rich, and others poor in abnor- 

 mal developments. One of the most notable instances is 

 the year 1851, in the summer of which pitcher-like leaves 

 appeared in such large numbers, as to be regarded as a dis- 

 tinct disease. This was the case in- the western districts 

 of Belgium, and while roses and other plants were notably 

 affected, the damage done to the tobacco cultures was re- 

 ported to be quite heavy. In Baden the summer of 1886 

 and around Paris the fall of 1893 are described as exception- 

 ally rich in abnormal productions, and other instances could 

 easily be given. Evidently some external cause was at 

 work, and high temperature and moisture are among the 

 factors called upon by different authors to explain the fact 

 which so deeply impressed them. 



In a single locality the same phenomenon may often be 

 observed. Some hillslopes and some fields are productive of 

 monstrosities, while others are not. By this means a bota- 

 nist is often enabled to discover the localities, which may 

 give him the deviations he wishes to collect. Sometimes it is 

 one species which offers a profusion of interesting instances; 

 in other cases a more or less large number of plants are 

 included in the phenomenon. This observation is often 

 stated by saying that a monstrosity is seldom seen alone; 

 it is quite generally accompanied by others. 



Some instances may be given as proofs. ' Pitchers are 

 ordinarily built out of one leaf, or of some part of it, but 



