INTRODUCTION. 1< 



typical of the adults of more primitive allies, but character- whirl, are n-ually eliminated in 

 development. Arrested variants are shown abundantly in tin- consideration of ocular plates, 

 when fewer plates may become insert than is characteristic of the -perie- ' Triplicate-, text- 

 figs. 123-125, p. 124). 



2. Progressive variation, in. which the variant has characters not typical of tl,, 



but which are further evolved on the direct line of differential development, and are -een typi- 

 cally in more evolved nearly allied species or genera. Such progressive variants are al-o -linwii 

 abundantly in the consideration of ocular plates, where more plates become in-ert than i- 

 typical in the species (Centrechinus, text-figs. 93-95, p. 107 and 176, p. 153,). 



3. Regressive variation, in which the variant takes on characters of the adult of .-ome >imple 

 and more primitive type of the group. Such characters are not necessarily a repetition of youth- 

 ful characters but may go back to a remote ancestry. An arrested variant in a sense is one form 

 of regressive variation, but a regressive variant includes much more than arrested variation. 

 To distinguish them, an arrested variant is one that has developed to a certain point as usual, 

 and then failed to take on the later added characters typical of the >pe<-ie-. >o that, although 

 an adult, it has immature characters. 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. A modern horse with extra digits as in Tertiary times could be 

 considered a regressive variant, but could not be considered an arrested variant. In Echini, 

 cases of regressive variation are shown in the simple ambulacrum in Meloiiechinus i Plate 57, 

 fig. 3), and in the single column of interambulacral plates in Arbacia (Plate 4, fig. 11) and Trip- 

 neustes (Plate 6, fig. 4). 



4. Parallel variation is where a character is taken on exceptionally which may be com- 

 pared with characters normally occurring in some type of the group not closely connei -led. -.. 

 that it cannot be genetically compared. A case apparently is the extra genital pores fre- 

 quently developed in Recent Echini (Echinus, text-fig. 115, p. 117). 



5. Aberrant variation is where a character is taken on which is (mite abnormal, not to 

 be correlated with the typical condition in associated forms. Cases of aberrant variation 

 are sea-urchins which have four or six areas developed (Plate 6, figs. 1-4). All the e\i.l 

 goes to show that aberrant variation is rare, and most variants can be considered a> am 

 progressive, regressive, or parallel variants, and as such can be correlated with >perie> more 

 or less nearly allied which typically possess the character which is a variant of the <-a>e in hand. 

 This holds true as far as known in both animals and plants, as seen in the present paper, and 

 as I showed in detailed studies of plants (1899). 



Next to stages in development, variation is the most suggestive line of study in attempt- 

 ing to work out the genetic relations of plants and animals. When a specimen of one species 



