314 Plant Life and Evolution 



not by any means necessarily follow, as accidents 

 may result in the destruction of the most vigorous 

 forms while the weaker survive to multiply their 

 kind. Thus in a lot of seedlings exposed to the 

 attacks of insects, birds, and other enemies, there 

 seems to be little discrimination, and the vigorous 

 young plants are perhaps even more likely to suffer 

 than the weaklings. Darwin assumes that there 

 may be developed as the result of natural selection 

 protective devices, as, for example, poisonous or of- 

 fensive secretions, which may render the plants dis- 

 tasteful, and of course it is the individuals in which 

 these characters are most pronounced which will 

 be most likely to survive. Unfortunately the dem- 

 onstration of this hypothesis is by no means easy. 

 There is no question about the variability of all 

 species. Not only does the same plant vary under 

 different conditions, but among young seedlings 

 growing under apparently exactly similar condi- 

 tions a large amount of variation can be observed, 

 and one must admit that the variation in many cases 

 must be considered as the result of individual pe- 

 culiarities, not to be explained as the result of any 

 evident environmental factors. Within the limits 

 of any species there may be a decided range of va- 

 riation, often showing a wide departure from what 

 may be called the mode or type. Such slight 

 differences are especially common in certain genera, 

 and are the source of endless confusion in attempt- 

 ing to define the limits of species. In Europe the 



