FUNCTION CHANGE. The disuse of an or gan for one function, and its modification for the performance of another; thus an organ may be transformed into another homologous with it, but performing a different function, serving a quite different use. It originates in a series of func tions performed by one and the same organ. Of these several functions one is the chief or pri mary, while the rest are secondary. If the primary function is for any reason suppressed, some one of the secondary functions becomes the chief one, and the final result of these processes is the transformation of the organ.
As an example may be mentioned the change of function in the anterior limbs of certain crus taceans from swimming and breathing uses to or gans of mastication_ (mandibles, maxilla:, and maxillipeds), the outer division, or `expodite,' undergoing reduction from disuse. Thus the original or chief function is suppressed, and what was an accessory or minor function becomes the chief one. More apparent examples are the change from the five-toed legs of the reptilian an cestor of birds into the wings, and of the fore legs of. the ancestors of whales into the paddles of existing cetaceans. All such changes of function are the result of change of environment, of .habits, and of instincts.
Still another good example of the principle of change of function is afforded by th6 swimming bladder of fishes. This in most fishes is a closed
sac lying directly beneath the backbone. In the gar-pike it has acquired a connection by a duct with the throat. It then becomes an accessory breathing organ in such fishes as the Protopterus of Africa, which is able temporarily to live out of water. Finally, by further change in habit and structure, this bladder with its pneumatic duct has become transformed into the lung of the amphibians, reptiles, and higher vertebrates. The transformation is due to change of surroundings and of habit, resulting in the changes of function.
This principle is pure Lamarekian doctrine; i.e. that changes of surroundings and of habits bring about changes of function or use, and finally of structure. Yet there are very numerous examples of this principle, and it has been most active in the origination of the classes and orders of ani mals.
BIBLIOGRAPHY. Saint George Mivart, Genesis of Bibliography. Saint George Mivart, Genesis of Species (New York, 1871) ; Darwin, Origin of Species (6th ed., London, 1882) ; A. Dohrn, Der Ursprung der Wirbelthiere und dos Princip des Punctionswechsels (Leipzig, 1875) ; A. M. Mar shall, Biological Lectures and Addresses (Lon. don, 1894).