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NEW YORK STATE MUSEUM 



instructive illustrations of several biogenetic principles evolved in the study 

 of other animal classes ; and they also give important evidence of the begin- 

 ning of a division of labor in the graptolite colony asserted before by the 



writer. 



Beecher in his philosophical paper " On the Origin and Significance of 

 Spines " l distinguishes a number of causes of the formation of spines. These 

 factors of spine genesis are divided in constructive and destructive agencies. 

 The former may arise from external stimuli or intrinsically from growtli 

 force. The latter extrinsically from external restraint or intrinsically from 

 a deficiency of growth force. 



The first group, which is the most important for our consideration, 

 offers five aspects under which the production and growth of spines may be 

 considered. Three of these, viz, growth (i) in response to stimuli from the 

 environment acting on the most exposed parts ; (2) as extreme results of 

 progressive differentiation of ornaments ; (3) secondarily as a means of 

 defense and offense, it is here pertinent to consider ; the two others (sexual 

 and mimetic influences) being excluded from consideration by the low 

 organization and our insufficient knowledge of the graptolites. 



There is no doubt that the spines of the great majority of the grapto- 

 lites have grown from the first mentioned cause and it is properly stated by 

 Beecher that " the apertural spines on some of the graptolites are on the 

 most exposed portions of the hydrothecae, as in Monograptus sp i nig- 

 erus, Dicranograptus nicholsoni, Retiograptus ten- 

 taculatus and Graptolithus quadrimucronatus." A com- 

 parison of the location of the spines in some of the genera will serve to 

 establish this point. In the great majority of the species the spines arc 

 formed at the apertures of the thecae, which project beyond the body of 

 the rhabdosome. They are there placed either singly on the outer side o( 

 the aperture, appearing as extensions of the "denticle" or paired and pro- 

 jecting from either side of the aperture as in Glossograpt us quad- 



'Beecher, C. E. Origin and Significance of Spines. A Study in Evolution. Am. 

 Jour. Sci. v. 6. 1898. 



