No. 490] ORTHOGENETIC VARIATION 



621 



half grown shells are absent or much more weakly developed, 

 than in the adult. 



In the same way lirae appear later in the life of individuals, 

 being absent from the earlier stages. They are furthermore often 

 complicated by intercalations, which like the primary ones appear 

 progressively. 



Summary OF Appearance of Sfieli. Ch ah a( tf.i^s. 



Summarizing the results so far obtained, we notr tliat all the 

 characters of the shell appear in a definite order, and develop by 

 a process of progressive intensification or growth. This law holds 

 good in the most diverse types of gastropods as will be more fully 

 apparent from the examples cited below as illustrations. Even 

 where at first glance the order of development seems to be abnormal, 

 it is seen on closer inspection that this apparent abnormality is due 

 either to normal progressive acceleration — falling as such under the 

 type of variation outlineMl in tlie next section, or to patholoiric con- 

 ditions, which cause a temporary, or sometimes permanent devel- 

 opment of senile characters wliicli in the normal (ourst" of the 

 ontogeny would appear only towards the end of life. 



Phyletic aecel.Tation. or tli.> con.lensation and eliminali.U! of 



lies^in the ontom-n.N \\hen'th<- elimination i^ at the \n'^uunu-^ of 

 the conch .ta^e. the diell mav be lookrd npon in the ^amc liglu as 

 a more primitiNe Uuc in uliich tlu> api. al portion has been broken 

 away. In both ( ases tlu^ available part begins some stages along 

 the road of prou'ress bur in both cases the stauvs stibsc(|ucntly 

 passed arc the s;im(\ ihoniili one mav uo at express rate and the 

 other at a dou pace When houever later .ta arr t,-l,..-opr<l. 

 as is theca.e i.i manv accelerated form. Tn/^rn <nrna. r ,l..nn., 

 Fusu,ln,urr,nuJu^. .cNeral .pe<-ie. of .sennfn.,;.. Pnuilina.rir the 

 recognition of the normal linr of pio^,, » i. n^t ... ,.a.N \,.x<m-- 



an.l from that point oi'i lard, thr projv.. i- a normal one mI-ivK- 

 an ori-inal .Irtonr ha. W-vu ent olf a. river in ihe <-onr..,. of its 



far apart on the river's conrs(> arc broniiht close touvther. 



