40 DIt. W. RAE SHERRIFFS ON EVOLUTION 



D. clavata with no canal-wall spicules belongs to what I would 

 call the more pi-imitive types. So careful a worker as Kiikenthal 

 attaches great importance to the canal-wall system of spicviles, 

 and it is obviously important to inquire what cori'elation there is 

 between the primitivenpss of a species and the number and 

 nature of the canal-wall spicules. The unsatisfactory feature 

 here is that the examination of the canal-wall spiculation has not 

 been made uniformly by the various investigators. Minute 

 spicules which escape attention in 

 a fragment of canal - wall when Text-fie-. 5. 



boiled down may be found by the 

 other method of examining a piece 

 intact. But one is not uniformly 

 successful with the sample examined. c^ad 



§ 8. Bearing in mind the first of the 

 above assumptions, it is possible to ex- 

 pand it so as to reduce all the difierent 

 species to one or other of the follow- 

 ing six grades, ranging from the most 

 primitive (I.) to the most specialized 

 (VI.) :- 



YI. Only one pair of spicules, or 



even btit a single spicule re- „ 



placing all the others in each 

 point above the crown ; 



V. reduction of the point that one 

 pair (or, it may be, one of the 

 uppermost pair) predominates 

 over the others, which are not 

 above three pairs in total 

 number above the ci'own ; /\ 



ik 



IV. one pair or one spicule predo- i- n j^\^ 



minating over 3-4 others, no ^— -^^ 



crown : j^^^^^ 



'is 



III. about 4-6 pairs, the uppermost 

 slightly specialized, no crown ; 



II. about 6-8 pairs, strong and 



uniform, no crown ; -Is 



• I. about 8-12 pairs in each poin t " f,\ 



numerous and small, rising /iU 



from indefiniteness to definite- \jjf 



ness, no crown. 



§ 9. The suggestion has been made by critics of Dendroneiph- 

 tliija species that the diflerences between the Glomerate, Divari- 

 cate, and Umbellate groups are not of taxonomic value, but 



