CHAPTER FIFTEEN 



VARIATION 



1. WE say that things are "as like as two peas," but Variability 

 two peas are not exactly alike. Everywhere among 



living beings we find variation ; the individuals of a 

 species differ in various ways from one another. Some 

 creatures are much more variable than others ; char- 

 acters which separate species in one group may only 

 distinguish individuals in another. Sometimes one 

 stage is more variable than another ; differently colored 

 caterpillars may produce a very uniform lot of moths, 

 as in the case of the white-lined sphinx. In other cases 

 the immature stages are very uniform, but the adults 

 vary. 1 Even when the variations are many and im- 

 portant, they follow certain lines, they are not indis- 

 criminate. Consequently, when a particular sort of 

 variety has been found in one species, we expect to see 

 similar variations in related species. What the cone- 

 flower has done, the sunflower will do. 



2. These variations, though all classed under one 

 general heading, really represent several quite different 

 phenomena. Theoretically we distinguish the follow- 

 ing : 



a. Variations due to changes in the germ plasm itself, Different 

 or "original variations." These may be due kinds ' of 



variation 



1 For beautiful illustrations of variations in caterpillars and moths, see 

 Packard's work on the Saturniidse, or great silk moths, in Memoirs National 

 Academy of Sciences, Vol. XII (First Memoir), 1914. For variation in 

 snails and slugs, see the colored plates in J. W. Taylor's Monograph of the 

 Land and Freshwater Mollusca of the British Isles, or H. A. Pilsbry's work 

 on Liguus, in Journal of Academy of Natural Sciences of Philadelphia, Vol. 

 XV (2d Series), 1912. All these works illustrate the subject in color in the 

 most exquisite manner. Any dealer in shells will supply series of Helix 

 nemoralis and various marine shells, illustrating variation. Leaves and 

 flowers (especially garden flowers) afford endless examples. 



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