CHAPTER V 



Natural Selection since Darwin (continued) 



The appearance of variations: their numerical importance; Del- 

 boeuf s law. — Frequency of variation. — Nature of variations; 

 usefulness and degree of development of certain characters. 

 — The neck of the giraffe and the femur of the whale. — 

 Abnormal development of characters. — Butterflies that 

 mimic leaves. — Parallel adaptation. — Overperfected organs. 

 — Analogy between natural selection and artificial selection. 

 — Secondary objections. — The true role of selection. 



AFTER reviewing the objections relative to the 

 modus operandi of natural selection, we come 

 now to two very important objections that bear 

 upon the very basis of operation of natural selection: 

 the appearance of variations and their hereditary 

 transmission. The first is worded as follows : Causes 

 of variation being weaker than causes of immuta- 

 bility the latter must necessarily prevail. 



According to Delboeuf's law, however small the 

 number of varying individuals compared with the 

 number of constant individuals may be, the varying 

 will increase steadily until they outnumber the con- 

 stant. This statement is based upon the following 

 calculations: Let us designate by A the initial type 



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