80 STUDIES IN EVOLUTION 



Genera having the ends only of the pleura directed backward 

 are generally less inclined to form spiniform terminations. 

 In contrast with these it is found that all the Trilobita 

 having the pleura directed outward, and with entire pygidial 

 margins, do not ordinarily develop long pleural spines; as 

 Asaphiis, Illcenus, Agnostus, Phacops, Calymmene^ etc. 



The examples of the caterpillars of moths belonging to the 

 Schizurse, described by Packard ^ as mimicking the serrations 

 of the leaves upon which they feed, have previously been 

 noticed in this essay, under the head of mimetic influences. 

 The initial cause of the spines may possibly be explained as 

 in part due to the mechanical conditions. During their early 

 existence the larvse feed on the lower side of the leaves, and 

 have no spines. Later they feed on the edges of the leaves, 

 and at the same time acquire dorsal spines. The conforma- 

 tion of the animal to the serrated edge of the leaf would 

 produce corresponding elevations and depressions on the back. 

 The location of these would be fairly constant from the habit 

 of the animal of feeding chiefly between the denser leaf veins 

 which determine and terminate the serrations. The raised 

 parts of the animal would receive the greatest amount of 

 stimuli, and at these points spines would naturally appear. 



The processes producing the spines noticed in this category 

 (IX) are classed with others under decrescence, for the reason 

 that the growth is restrained or controlled by mechanical 

 necessities. If the restraint were absent, it is probable that 

 a more expansive growth would take place or that other 

 structures would be correspondingly benefited. 



X. Disuse. (Cs, Do.) 



In causing the reduction or atrophy of an organ, the effects 

 of disuse have generally been recognized by most observers. 

 In this way the origin of many of the so-called " rudimentary 

 organs " has been satisfactorily explained by Darwin ^^ and 

 others. Two classes of structures are evidently comprised 

 within the common definition of rudimentary organs ; namely, 

 nascent and vestigial organs. 



