0. E. Beecher — Origin and Significance of Spines. 339 



Likewise, if the pleura or their distal ends are directed poste- 

 riorly nearly parallel to the axis, the mechanical necessities of 

 motion require that the portions of the free segments pointing 

 backward should be free, thus making the ends of the thoracic 

 pleura generally appear as retrally curved spiniform extensions. 

 Extreme examples of retrally directed pleura accompanied by 

 small pygidia are shown in Paradoxides (figure 65), Holmia, 

 Olenellus, Elliptocephala, etc. 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 Trilobites having the pleura directed out- 

 wards and with entire pygidial margins, do not ordinarily 

 develop long pleural spines ; as Asaphus, Illce?ius, Agnostics, 

 P/iacops, Calymene, etc. 



The examples of the caterpillars of moths belonging to the 

 Schizurge, described by Packard 54 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 larvae 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 conformation 

 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 struc- 

 tures would be correspondingly benefited. 



X. Disuse. (C„ D 2 ). 



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' 4 and 

 others. Two classes of structures are evidently comprised 

 within the common definition of rudimentary organs, namely, 

 nascent and vestigial organs. 



Nascent structures indicate the beginnings or initial stages 



