34 STUDIES IN EVOLUTION 



responsive action between growth and stimulus, which, as 

 ah-eady seen, tends to produce a conical or spiniform growth. 



Under ordinary favorable conditions, simple external stimuli 

 acting blindly through no agencies of selection would develop 

 spines on all the most exposed parts, and tend to differentiate 

 ornamental features. This has been the case with many 

 organisms and colonial aggregates possessing no power of 

 selection or not acted upon by any forces of determination, 

 conscious or unconscious. In such cases spines may or may 

 not serve for protection, and their function, if any, can be 

 only determined separately for each case. If, however, the 

 added function of offence is included, it is manifest that the 

 sjDines must be located in special positions adapted to use for 

 offensive purposes, as on the tails of some animals, and not 

 necessarily over vulnerable parts. Here the selective agency 

 of special adaptation is shown. Again, if while there is 

 agreement in other essential characters, spines or horns are 

 confined to either sex, it is evidently a case of sexual selec- 

 tion. Further, if they develop in harmony with the environ- 

 ment, or in a manner parallel to similar features of other 

 organisms, it is through the operation of physical selection. 



Altogether, under the general forces of external stimuli, 

 there are five aspects in which to consider the production and 

 growth of spines ; namely, 



A. From External Stimuli. 



A 1. — In response to stimuli from the environment acting 

 on the most exposed parts. 



A 2. — As extreme results of progressive differentiation of 

 ornaments. 



A 3. — Secondarily as a means of defence and offence. 



A 4. — Secondarily from sexual selection. 



A 5. — Secondarily from mimetic influences. 



B. Growth Force. 



In unicellular organisms growth force, or bathmetic energy, 

 must reside wholly in the germ cell, and therefore is con- 

 cerned with reproduction as well as Avith cell differentiation. 



