22 STUDIES IN EVOLUTION 



small antlers, which are seldom branched, and become thick- 

 ened by irregular deposits of bone (Owen ^3). Spines grow 

 during the adolescence of the Horseshoe Crab, Limulus 

 polyphemus^ yet in old age they are obsolescent, being repre- 

 sented by rounded nodes. 



As examples illustrating the accelerated development of 

 spines in widely separated classes, the Giraffe among mam- 

 mals and Acidaspis among Arthropoda may be selected. The 

 Giraffe represents the continuance of a very primitive type of 

 horn ; namely, one covered with a hairy skin. They are never 

 shed, and are common to both sexes. Out of this type all 

 others found among the Mammalia have probably been devel- 

 oped. The point of interest here is that the young Giraffe is 

 born with horns, and as these could serve no prenatal purpose, 

 it must be concluded that the action of accelerated heredity 

 has pushed the development of these organs so far forward 

 as to cause them to appear during fcetal growth. 



The next illustration of acceleration is taken from the Trilo- 

 bita. Acidaspis is one of the most highly specialized and 

 ornate genera. Although the larval forms of other genera are 

 commonly without ornament, yet in the present genus the 

 protaspis, or phylembryonic, stage partakes of this specializa- 

 tion in so far as to develop minute spines, which later become 

 larger, more differentiated, and form a conspicuous feature of 

 the adult. Other characters have been likewise shown to 

 appear at an earlier period than in other genera, and the earlier 

 inheritance of spines must be explained in the same manner.^ 



The facts, as stated, seem to warrant the conclusion, that in 

 spinose organisms the very young are almost universally with- 

 out spines. Acceleration may occasionally push their devel- 

 opment into the embryonic and larval stages, but ordinarily 

 they are not so subject to the action of this law as are some of 

 the physiological and other structural characters. This will be 

 explained as in part due to the lack of general plasticity, and 

 because differentiated spine growth is the progressive limit of 

 variation. Therefore there are no subsequent characters to 

 displace them and crowd them forward in the ontogeny. 



