CXX11 INTEODUCTION. 



and clearly discriminating, the more constant and perma- 

 nent forms, with due attention to their evolutional his- 

 tory and varietal tendencies, than by merging them in 

 genealogical groups the composition of which must, in 

 many cases, be largely speculative. 



One of the most important features of Prof. Smitt's 

 system is the place which he assigns to the zooecium in 

 the construction of families and genera. The mere mode 

 of growth he treats as a perfectly subordinate character, 

 and bases his divisions chiefly on the essential element 

 of the structure, the cell *. This revolutionary step in- 

 volves the breaking-up of a large proportion of the older 

 genera and the wide dispersion of forms hitherto most 

 closely associated ; but there can be no doubt that it is a 

 necessary preliminary to the introduction of a natural 

 system. The varieties of habit which have been made 

 the criteria of genera, may occur within the limits of a 

 species. It is not the mode in which the cells combine, 

 but the cell itself, that is the true test of relationship 

 and the essential basis of a natural group. Prof. Smitt 

 has remarked, "Semper videbimus, ubi forma zooecii eadem 

 sit, ceteras dissimilitudines altero vel altero modo evane- 

 scere." 



Retaining Mr. Busk's three principal divisions of the 

 Gymnol&mata, he arranges the Cheilostomata under four 

 groups : the Flustrina, with quadrate cells, the front sur- 

 face of which is flat and equals the area of the primitive 

 aperture; the Cellularina, with corneous or corneo-calca- 

 reous inftmdibuliform cells, the inferior portion of which 



* Of course, in practice, this principle applies chiefly to the Cheilosto- 

 mata. Amongst the Cyclosfomata there is great uniformity of structuro 

 in the zooecium ; and in the owe of tin- {.'ti'ii<'*l<>ni<ifu other important dm 

 racters are available. 



