212 GENERAL BIOLOGY 



increases the variability of organisms. Without 

 doubt a greater range of characters is permitted to 

 survive by the florist or animal-breeder than would 

 be found in a state of nature, but the innate capacity 

 for varying, the true variability of the species, does 

 not appear to be altered. 



In some way not understood the climate of a 

 region has a marked influence upon the variability 

 of the plants and animals inhabiting such a region. 

 A botanist has compared some twenty-nine kinds of 

 trees grown in America and Europe under practically 

 the same conditions. His results are quoted by 

 Darwin as follows : "In the American species he 

 finds, with the rarest exceptions, that the leaves fall 

 earlier in the season, and assume a brighter tint 

 before they fall ; that they are less deeply toothed 

 or serrated ; that the buds are smaller ; that the 

 trees are more diffuse in growth and have fewer 

 branchlets ; and lastly, that the seeds are smaller, 

 all in comparison with the corresponding English 

 species." 



Causes of Variation. Variations may be of two 

 kinds : those that are induced by the direct action 

 of the surroundings and those that arise sponta- 

 neously " from within " the germinal substance. 

 The former are called somatogenic, inasmuch as they 

 affect only the soma. As we shall see a little farther 

 along, such characters affect the individual without 

 being " handed on " to its posterity, and hence, 

 from the standpoint of the race, are transitory in 



