No. 543] BIO TYPES AND PHYLOGENY 



L45 



even in a polygenoplastic population receives an added 

 indorsement. 



A third problem of the systematist (and for this occa- 

 sion the last) is found in the fact that diversity of mor- 

 phological characters in any given species is not hap- 

 hazard or indiscriminate, but is generally restricted to 

 such definite lines as to indicate more or less distinct 

 stages in the phylogenesis of that species. The belief that 

 diversity is significant and that its meaning may be dis- 

 covered has received extraordinary confirmation in Jack- 

 son's just published, magnificent monograph on Echini 2 

 in which the subject is very fully discussed. An illustra- 

 tion taken from his work will help to make clear the de- 

 sired point. In any regular sea-urchin, such as Arbacia 

 or Sjrongyloccntrotus, a group of ten plates surrounds 

 the periproct, five of which are radial in position and are 

 called oculars while the other five are interradial and are 

 called genitals. Now in some echini all of these ten plates 

 are in contact with the periproct and thus form a simple 

 continuous ring but in most of the Eecent species, the 

 oculars are much smaller than the genitals and some or 

 all of them are separated from the periproct by the meet- 

 ing of adjoining genitals. In other words, some of the 

 oculars may be excluded from the periproct and such are 

 said to be exsert, while those which separate adjoining 

 genitals and reach the periproct are called insert. Now 

 Jackson has demonstrated conclusively, contrary to the 

 widely held belief that the insertness of oculars is a mat- 

 ter of age and size, that for each species of sea-urchin 

 there is a characteristic arrangement of the genito-ocular 

 ring and that this arrangement is oftentimes a very con- 

 stant character. Thus in 2,100 Arbacias from Woods 

 Hole, 87 per cent, have all the oculars exsert and in more 

 than 20,000 Strongylocentroti from Maine 95 per cent, 

 have the two posterior oculars insert. 



Having demonstrated the constancy of this character, 

 Jackson has gone on to an analysis of the variations from 



