46 STUDIES IN EVOLUTION 



Such spines represent periodic extensions of the mantle 

 border, and in some cases the stimulus for this growth may- 

 come from internal causes. The spines on Unio spinosus and 

 related species are believed by Mr. Charles T. Simpson to 

 assist in anchoring the shell in the sand of swift running 

 streams. In Callista, the young Saxicava, and the Unios 

 mentioned, the spines occur on all individuals and at such an 

 early period as to preclude any special sexual function. 



In the Gastropoda the periodic extension of the shell over 

 the posterior canal and the spiniform prominences formed 

 on the labrum are situated in exposed places, or where the 

 amount of stimulus is greatest; as in Trophon magellanicus^ 

 Strombus pugilis^ Fusus colus, Clavatula mitra, Melo dia- 

 dema, etc. 



The spines on the larvae of geometrid moths are usually on 

 top of the loop, and are explained by Packard ^^ as follows : 

 " The humps or horns arise from the most prominent portions 

 of the body, at the point where the body is most exposed to 

 external stimuli." 



When the origin and function of spines in a great many 

 forms of animals, and especially among the higher classes, 

 are examined, it seems almost impossible to decide whether a 

 spine has been originated and perpetuated by free variation 

 and heredity, or by the general action of external stimuli 

 on the most exposed parts; and in the latter case, whether 

 or not under the selective influences of use. Its origin in 

 either instance may be through external stimuli, but in the 

 latter, it falls under other captions than Ai; or, in other 

 words, the external stimuli excite the growth force at certain 

 points, and the growths so produced may be simply reciprocal 

 without function or they may serve purposes of protection or 

 offence. Thus the dorsal and rostral spines on the zoea of 

 the Decapoda are on the most exposed points, and seem 

 to function as defensive structures. As soon as the legs 

 become well developed or when the animal ceases to swim at 

 the surface and hides among the stones, etc., at the bottom, 

 these spines become reduced and are often succeeded by 



