C. E. Beecher — Origin and Significance of Spines. 253 



" 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 varia- 

 tion 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, bat 

 in the latter, it falls under other captions than A,; 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 pro- 

 tection or offense. Thus, the dorsal and rostral spines on the 

 zoea of the Decapods 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 others. 

 The spines of the adult are also usually efficient for protection, 

 but owing 1 to the change in form of the animal and change of 

 habitat, the most exposed parts are different from those of the 

 larva, and the spines are frequently developed where there 

 were no larval spines ; as in Cancer irroratm, Callinectes has- 

 tatus, etc. Again, the horned ungulates show in their habits 

 of sport, fighting, defense, and procuration of food, that the 

 exposed angles of the top of the skull are subject to the 

 greatest number of stimuli, and there the horns are developed. 

 The connection between external stimuli and growth is here 

 most manifest, for it is impossible to imagine the action of free 

 variation or simple growth force as resulting independently, 

 in the evolution of horned out of hornless species in several 

 suborders of mammals, and in every case determining the loca- 

 tion of the horns on the prominent angles of the skull, whether 

 on the nasals, maxillaries, frontals, or parietals. 



It is well known that toads and frogs defend themselves by 

 using the head as a shield, and the cranial angles thus receive 

 the greatest amount of stimulus. " There are natural series of 

 genera measured by the degree of ossification of the superior 

 cranial walls " (Cope 10 ). In the highest genera, the head is 

 completely encased, and in some forms the projecting angles 

 are developed into short horns. The so-called " Horned Toad" 

 {Phrynosoma) has the same habit of defense, and it is believed 

 that this mode of protection or of receiving impacts has given 

 rise to the structure, by stimulating growth at these points. 



