16 STUDIES IN EVOLUTION 



tures appearing late in the ontogeny of the individual 

 will present primitive infantile and adolescent characters. 

 Further development, if such takes place, will pass through 

 a progressive series of ontogenetic changes, and if the stages 

 of growth are by serial repetition and thus are retained in 

 the part, it will be found that such stages can be correlated 

 with those appearing early in the life or history of the indi- 

 vidual. Therefore, in studies of this kind, it is possible to 

 take a structure appearing at maturity, and from it deduce or 

 predicate as to what were some of the early characteristics 

 of the whole individual. This principle is termed localized 

 stages of growth by Jackson, ^'^ and was first noticed by him 

 in some investigations on Echinodermata. 



Compound Spines. 



A simple, sharp, conical process expresses only the primi- 

 tive type of spine. In plants and animals it is the most 



21 22 



FiGUKE 21. — Simple spine. 



FiGDEE 22. — Spine, with lateral spinules. 



FiGDEE 23. — Spine, with forked apex and lateral spinulose spinules. 



common form found, and is the first stage of spine differen- 

 tiation. From this type the myriad forms of spines known 

 in the organic world are produced by almost insensible 

 gradations. It is needless to attempt a detailed description 

 of this infinite variety; but, as a single illustration, some of 

 the leading forms of spine differentiation among the Radio- 

 laria are here shown (Plate I). These figures are taken 

 from Haeckel's "Report on the Radiolaria, " ^^ and generally 

 represent enlargements of from 100 to 400 diameters. Prob- 

 ably no other class of organisms presents greater variety, and 



