38 STUDIES IN EVOLUTION 



duced through growth force, the following factors are offered 

 for consideration: 



B. From Growth Force. 



B 1. — Prolonged development under conditions favorable 

 for multiplication. 



B 2. — By repetition. 



B 3. — Progressive differentiation of previous structures. 



B 4. — Secondary development through the selective influ- 

 ences of defence, offence, sex, mimicry, and other external 

 demands. 



C. External Restraint. 



Intermittent stimulus, as previously shown, produces 

 growth in the direction of function. When the growth 

 equals the waste, an equilibrium or static condition is 

 reached, and no relative change occurs. The absence of 

 either extrinsic or intrinsic stimulus will not be favorable 

 to growth, and under such conditions an organ or struc- 

 ture may remain undeveloped, or, if already present in the 

 organism, it may waste away and degenerate into a vestigial 

 structure, or even disappear altogether. 



On the other hand, it is well known that continuous pres- 

 sure not only prevents growth, but in addition resorption 

 takes place, and in this way the whole or a portion of a 

 structure may be removed. These changes have frequently 

 been studied in embryos, as well as in many internal struc- 

 tures, and are also familiar in the enlarged pedicle-openings 

 of many Brachiopoda, caused by pressure of the pedicle, 

 and in the similar opening for the byssal plug of Anomia. 

 Packard ^^ gives examples among the Crustacea and Insecta, 

 which are clearly to the point. He says of the Crustacea, 

 "It may here be noted that the results of the hypertrophy 

 and overgrowth of the two consolidated tergites of the second 

 antennal and mandibular segments of the Decapod Crustacea, 

 by which the carapace has been produced, has resulted in a 

 constant pressure on the dorsal arches of the succeeding five 



