68 STUDIES IN EVOLUTION 



Carrying this repetitionary process to a maximum, there 

 would result a generally spinous condition. As a possible 

 illustration of this, no class of organisms probably exhibits so 

 many kinds and series of repetitions of all sorts of external 

 structures as the Echinodermata, and it is significant that 

 this is a typically spiniferous sub-kingdom. 



Except in a few classes of organisms, compound spines are 

 relatively rare as compared with simple spines. They are 

 very common among the Radiolaria, which furnish the 

 greatest complexity occurring anywhere in the organic 

 world. (See Plate I.) They are also quite frequent among 

 the Echinoidea, but more rare among the Asteroidea and 

 Crinoidea. 



Compound antlers are especially characteristic of the mod- 

 ern Deer family, though compound horns are but rarely 

 found elsewhere among the mammals. The Prong-horn 

 Antelope of America is the only living species of hollow- 

 horned ruminant having this character. It of course is not 

 intended that extra pairs of horns, which being separate, and 

 often originating on different portions of the skull, should be 

 considered as compound horns in the sense here employed. 

 Likewise compound spines arising through suppression of 

 organs or structures are not to be included here; as the 

 compound thorns on the Honey-locust representing aborted 

 branches. 



The fin spines of fishes are often compound, and sometimes 

 are made up of several elements ; as in the spines of Edestus 

 (^E. vorax). Quite a number of Mollusca develop compound 

 spines ; as in many species of Sjjond^lus and Murex. They 

 are also not uncommon among the Crustacea and Insecta. 

 Compound spines are infrequent in the Brachiopoda, being 

 developed in but few species (^Spirifer hirtus ^^). The 

 Foraminifera also present but few examples (^Polymorphina 

 Orhignii ^). 



A number of generally or highly spinose types will now 

 be noted to illustrate the limits of the repetition of spiny 

 structures, the first spines having probably arisen through 



