Schuchert — Jackson on the Phytogeny of the Echini. 253 



"In order to appreciate variation it is of fundamental 

 importance to be familiar with the characters of the associated 

 species and genera of a case in hand, and also the develop- 

 mental characters of the same. Variation may be fairly clas- 

 sified under five more or less distinct heads : 



" 1. Arrested variation, in which the variant retains char- 

 acters seen in its own young and typical of the adults of more 

 primitive allies, but characters which are usually eliminated in 

 development. . . . 



" 2. Progressive variation, in which the variant has char- 

 acters not typical of the species, but which are further evolved 

 on the direct line of differential development, and are seen 

 typically in more evolved nearly allied species or genera. . . 



" 3. Regressive variation, in which the variant takes on 

 characters of the adult of some simple and more primitive 

 type of the group. Such characters are not necessarily a repe- 

 tition of youthful characters but may go back to a remote 

 ancestry. An arrested variant in a sense is one form of regres- 

 sive variation, but a regressive variant includes much more 

 than arrested variation. To distinguish them, an arrested 

 variant is one that lias developed to a certain point as usual, 

 and then failed to take on the later added characters typical of 

 the species, so that, although an adult, it has immature char- 

 acters. A regressive variant is one that has attained full char- 

 acters and then in later life has reverted to youthful or 

 primitive characters as an individual variation, or it is a variant 

 that from youth has primitive characters not normally seen in 

 the development of the species. 



" 4. Parallel variation is where a character is taken on 

 exceptionally which may be compared with characters nor- 

 mally occurring in some type of the group not closely con- 

 nected, so that it cannot be genetically compared. 



" 5. Aberrant variation is where a character is taken on 

 which is quite abnormal, not to be correlated with the typical 

 condition in associated forms" (pp. 18, 19). 



Comparative Morphology. 



Significance of abnormal symmetry. — Echini are remarkably 

 constant in their pentamerous system, but Jackson found 71 

 variant individuals or on an average " a little more than one 

 to a thousand. The variants are partially or completely tri- 

 merous, tetramerous, and hexamerous. . . . The ocular 

 plates seem to exert a controlling influence in the building up 

 of the corona, as below and in immediate contact with the 

 oculars originate the coronal plates, both ambulacral and inter- 

 ambulacral. In connection with each ocular is developed a 

 whole ambulacrum, and, in addition^ a half-interambulacrum 



