No. 490] 



ORTHOGEXETIC YARIA TION 



639 



INIelanias in Ceritbiuni, Turritella, etc., onuuiientatioMs aj)])ear in 

 the young which are lost in the achilt. A^ itliout <le])artm<4' from an 

 orthogenetic mode of development, the succession of characters 

 developed seems to be inverted. In such cnscs it u'ciicrallv ai)j)cars 

 on study of the youngest stages that the simple cliaracicrs of tiie 

 adult are similar to the most youthful cliaracicrs. Ix torc ilic most 

 pronounced features appear. This featuit^ will he dcsciihcd later 

 m some detail in certain Melanias and is also seen in other special- 

 ized types. It is readily explained by refcrriiiir it to dcuradational 

 development, where characters accpiired diirniu' ilie a::iira(latioiial 

 period are lost in the reverse order of their acquisition, ^oinetunes 

 differentia] acceleration may account in part for this, riius m 

 some cases, the shell becomes carinate and only subseciuentlv 

 develops ribs. These may be a later acquisition luivmo; never 

 before appeared in the phylogeny of thi.s series. ^^ hatever the 

 cause of such develo])nient, the method is orthogenetic, the 

 variation being in all cases {)ro<;ressive in one dii-ection or another. 



Examples Illustrating Orthogkxksis. 

 I have elsewhere described a con.si(leraV)le number of genetic 



It has there been clearly shown that the developnieiit i> orthoge- 

 netic, both as regards phylogeny and ontogeny, and that the great 

 governing principle in the production of diversity is acceleration or 

 tachygenesis, and retardation or bradygenesi^. To >liow more 

 clearly the universality of this |)rinciple of orthogenetic variation 

 among the Gastropoda, I will here ai)j)end a somewhat detailed 

 di.scussion of several phyletic lines among the Melanias. a i^roup as 

 di.stinct as possible from the Fusidae. unfortunately in the speci- 

 mens stu.lied, the protoconchs were not available, .so that all the 

 deductions are bast^l U})()n the characters ot the conch, from its 



Melania is well adapted to this sort of study on account not only 

 of its variability, but also l)ecaiise it has so many characters upon 



Uoc. cit. 



