Schuchert — Jackson on the Phylogeny of the Echini. 257 



frequently lost in fossils. I have succeeded, however, in mak- 

 ing observations on something over 50,000 regular Recent and 

 Mesozoic specimens representing 133 species. . . . The reason 

 for making so many observations was that while the character 

 of a species is usually gathered correctly from five or ten 

 specimens, the variations seen in a large number present inter- 

 esting data for comparative study. . . 



" In Mesozoic regular Echini the dominant character is for 

 all the oculars to be exsert, or excluded from the periproct. In 

 the Recent regular Echini the young also have all the oculars 

 exsert. In the adult all the oculars may be exsert or one or 

 more be insert. While the exsert character of the young is 

 like the Mesozoic, the becoming insert in development is the 

 taking on of a character which in this respect is directly com- 

 parable to the dominant character of the Palaeozoic. . . . 



" As becoming insert is a progressive character with develop- 

 ment, species in a genus that have the greatest number of 

 ocular plates insert may be considered in this respect more 

 evolved than other species which have a less number (Arbacia, 

 Echinometra). Also, as a matter of variation, individuals that 

 have fewer oculars insert than is characteristic of the species 

 may be considered arrested variants, and those that have more 

 plates insert than is typical may be considered progressive 

 variants. Such variants can frequently be compared directly 

 with related species or genera where the fewer or greater num- 

 ber of oculars insert is a typical specific character (Cen- 

 trechinus). Specimens of a given species from different local- 

 ities present often quite striking differences as regards the 

 number of plates which are insert, those from one locality 

 having typically fewer oculars insert than those from a differ- 

 ent locality. Such variation with locality may well be con- 

 sidered as indicating incipient species, as, where there is a dif- 

 ference, specimens from one locality must be more progressive 

 or less progressive than those from another. . . . 



'' The number of oculars insert has been spoken of by pre- 

 vious writers as if it were a concurrent of age, and the largest 

 specimens had the most oculars insert. My observations are 

 directly opposed to this view. All the evidence goes to show 

 that the full number of oculars that are to become insert are 

 developed early in the life of the individual, and apparently 

 later no change in this respect takes place. A series of 

 specimens half the mature size or larger may in most species 

 be safely accepted as showing the mature characters as regards 

 oculars. This is on the basis of observations on 11,500 

 specimens of Strongylocentrotus drobachiensis, all from one 

 locality, Dumpling Islands, the specimens varying from very 

 young to adult, and all measured and tabulated as later 



