56 niOCEEDlNGS OF TUB 



"Jordan's species" which aiv in()rphoh)gieally distinct. The size 

 and shape of the spores oi' the •'elementary species " show two 

 kinds ot variation : (1) noimud variation, wliich is always present 

 upon whatever host or substratum the Fungus develops, aud the 

 range of whicli is characteristic of the elementary species ; and 

 (2) " modal variation,'' which is the variation in the size and shape 

 of the mode-spore, and is directly and constantly induced by the 

 particular substratum. In nature the "elementary species "are 

 usuallv found upon particular hosts, but they readily attack other 

 plants and, when growing saprophytically, are omnivorous. 



The size and shape of the spores of the Fungus growing in 

 nature are therefore not morphological constants but resultants of 

 the strain of the fungus and the substratum upon which it is 

 developing; and the only method of critically identifying the 

 particular elementary species present is to isolate it in pure culture 

 and obtain its " modal variation " upon a series of standardized 

 media. 



The presence of modal variation in the size and shape of the 

 spores has been ascertained in three species of Penicillium and one 

 species of Sti/sanits which have been investigated, and it is 

 suggested that it may be of general occurrence in the Fungi. 



Exhibition of lantern-slides representing a series of intermediate 

 forms of the Diatom genera Navicula and Ci/mhella, by 

 Sir N. Yermoloff, K.C.B., K.C.V.O. (Counnunicated by 

 Dr. A. B. Eexdlb, F.R.S., Sec.L.S.) 



[•20fh June, 1918.] 



The lantern-slides will show a series of closely connected forms, 

 so gradually and continuously passing into one another, that it 

 would seem possible to group them into one synthetic genealogical 

 evolutionary line of descent. A fossil Diatom from the freshwater 

 deposit of 'Monmouth, in the American State of Maine, lying 

 approximately at the latitude of the Great Canadian Lakes — this 

 deposit probably dating from before the Glacial periods, — may be 

 considered as the primordial ancestral form from which the whole 

 series has derived during the post-GIacial ages. This parental 

 form is Navicula moamoutluana forma gennina. 



The idea of grouping together into lines of genealogical descent 

 closely connected intermediate forms of Diatoms, which, in the 

 enormously crowded Systematics of Diatomaceae, may otherwise 

 stand rather far apart, has been hinted at by Cleve. The present 

 attempt at what may be called Synthetic Integration of inter- 

 mediate forms has been undertaken in further pursuance of 

 Cleve's idea. The work mainly consisted in finding, between 

 firmly-defined species, the connecting links. 



