No. 543] 



BIOTYPES AND PHYLOGENY 



147 



sense, accepted by Jackson, includes more than twenty 

 species. Of these some have the ambulacra relatively 

 simple, the compound plates being made up of only four 

 or five elements each, while in the more specialized spe- 

 cies there may be as many as ten elements in each com- 

 pound plate. The various species can be arranged 

 roughly in a series beginning with the simplest and end- 

 ing with the most specialized 3 and Jackson shows that 

 the species with the simplest ambulacra (8. lividus) 

 has "no oculars insert" as the species character, with 

 "right posterior ocular insert" as a common variant, 

 while those with the most complex ambulacra (S. fran- 

 ciscanus and purpuratus) have two and often three 

 oculars insert. Now in our common Strongylocentro- 

 tus from Maine, while practically 95 per cent, have two 

 oculars insert, nearly 3 per cent, have only one insert, as 

 in the common variant of S. lividus, while about 2 per 

 cent, have three insert as in the usual variants of S. pur- 

 puratus. Jackson calls these arrested and progressive 

 variants, respectively, according to whether they resem- 

 ble a more simple or a more complex allied species. 

 Whether the terminology be accepted or not, the signifi- 

 cance of such facts can not be ignored. Are we any 

 better prepared, with our present knowledge of the ex- 

 istence of biotypes, to understand the reason for this 

 significance of variation? 



If we compare a polygenoplastic group with a highly 

 complex chemical compound, an analogy is suggested 

 which warrants our answering this question affirma- 

 tively. In building up such a compound synthetically, 

 the specific properties of the constituents result in the 

 formation of certain definite compounds. These sub- 

 stances are necessary for the further combinations 

 without which the ultimate compound could not be 

 formed. In other words, the formation of the desired 

 product is possible only because the chemical reactions 



* There are some interesting exceptions, but as they do not affect the 

 subsequent argument, they need not be discussed here. 



