THEORETICAL BIOLOGY 



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between individuals possessing the selected characteristics and 

 those in which they were absent or imperfectly developed, and 

 thus they would gradually tend to disappear. This result has 

 been brought about by a sort of cessation of natural selection, 

 and the process is known technically as panmixia. We may 



FlG. 425. Methona psidii, upper figure, a butterfly distasteful to birds, imitated by Leptahs 

 orise, the lower figure. (After Wallace.) 



note as an illustration of this process an example already con- 

 sidered as illustrating Lamarck's theory of use and disuse, thus 

 showing how the two theories account for the same phenomenon. 

 As the ancestral Cetacea sought the land from time to time, 

 posterior appendages were an advantage to them ; but as they 

 became entirely aquatic, such structures were not only useless 

 but a positive hindrance to locomotion, and so interfered with 

 many of the functions of life. Hence individuals which ex- 



