ORIGIN AND SIGNIFICANCE OF SPINES 39 



cephalic and five thoracic segments, until as a result we have 

 an atrophy of the dorsal arches of as many as ten segments, 

 these being covered by the carapace." 



The restraint of the environment through unfavorable con- 

 ditions is the antithesis of A, or the influence of constructive 

 external stimuli, and is considered as the extrinsic operation 

 of destructive agencies. It is evident that external unfavor- 

 able conditions will repress growth, with a resultant atrophy 

 of the structures affected. In this way, also, the environment 

 may cause the disuse of an organ, which by consequent sup- 

 pression may dwindle away to a spine, as in the leaves and 

 branches of desert plants, and the spurs of the Python ^^ 

 representing the hind limbs. It may likewise repress growth, 

 as in the spines on the lower side of the poriferous coral 

 Michelhiia favosa,'^^ representing aborted attempts at bud- 

 ding, the failure being due to the unfavorable position of the 

 buds for securing food. 



The restraint of the environment may also act in a mechan- 

 ical manner to produce spines, as will be shown subsequently 

 in some Brachiopoda and Trilobita. Furthermore, spines 

 arising through any phase of external restraint, may second- 

 arily come under the influences of natural selection, and be 

 useful for protection and offence, or conform to other external 

 demands. 



Under the head of external restraint, therefore, are the 

 followir.g categories: 



C. From External Restraint. 



CI. — Restraint of environment causing suppression of 

 structures. 



C 2. — Mechanical restraint. 



C 3. — Disuse. 



C 4. — Secondarily for protection, offence, etc. 



D. Deficiency of Growth Force. 



The growth force in organisms may be reduced in several 

 ways, the most general and obvious modes being by an 



