ORIGIX A^'D SIGNIFICANCE OF SPINES 31 



thousand species are known, and of these probably not more 

 than one hundred and fifty are living. 



Similar data are derived from the Trilobita. This group 

 is found all through the Paleozoic, at the close of which it 

 became extinct. Two of the three orders are found in the 

 Lower Cambrian. The remaining order appeared just after 

 the close of the Cambrian in the early Ordovician, yet 

 through the whole of the remaining sediments not a single 

 new ordinal type was developed. When applied to a single 

 order, the same truth comes out. The order Proparia is 

 one whose entire history can be traced, extending from the 

 Ordovician through the Silurian and Devonian. All the 

 families appear in the Ordovician ; in fact not a single family 

 type in this or the other orders was produced during the 

 whole Silurian, Devonian, and Carboniferous.^ 



As the classes, orders, and families are based upon the 

 physiological and important functional structural characters 

 or differences, it is evident that at or near the beginnings of 

 their life history is found the demonstration of the domina- 

 tion of phylogenic over ontogenic characters. 



Conditions or Forces affecting Growth. — Since spines are 

 purely organic structures, their production must follow the 

 general laws of organic change. The forces considered as of 

 most consequence are two : (1) the external stimuli from the 

 environment, and (2) the energy of growth force. These, 

 with their opposites (1 a) the restraint of the environment, 

 and (2 a) the deficiency of growth force, are believed to 

 include the chief active and passive causes, not only of spine 

 production, but of growth and decline in general. Correlat- 

 ing these four causes with their constructive and destructive 

 agencies, together with their extrinsic and intrinsic modes of 

 action, as previously explained, there result (A) the external 

 stimuli of the environment as an extrinsic cause of concres- 

 cence; (B) energy of growth force as an intrinsic cause of 

 concrescence; (C) external restraint as an extrinsic cause 

 of decrescence ; and (D) deficiency of energy of growth force 

 as an intrinsic cause of decrescence. The remaining vital 



