ORIGIN AND SIGNIFICANCE OF SPINES 47 



others. The spines of the adult are also usually efficient for 

 protection, but owing 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 Oancer 

 irroratus, Callinectes hastatus, etc. Again, the horned ungu- 

 lates show in their habits of sport, fighting, defence, 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 sub-orders of mammals, and 

 in every case determining the location 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^"). In the highest genera the 

 head is completely encased, and in some forms the project- 

 ing angles are developed into short horns. The so-called 

 " Homed Toad " QPhrynosoma) has the same habit of defence, 

 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. 



II. As extreme results of progressive differentiation of 

 previous structures. (Aj, B3.) 



The differentiation of existing ornamental structures into 

 spines has already been noticed in several instances in this 

 article. It was shown that spines often arise by the elonga- 

 tion of nodes and tubercles or similar structures, by rhythmic 

 alternating areas of growth in lamellae and ridges, and by 



