194 PROCEEDINGS OP THE 



Oligocene period, when Europe suffered great oceanic invasions, 

 one from the Arctic seas towards Denmark, and another from 

 southern waters northwards towards the latitude Riga-Simbirsk, 

 At the same time the seas covering Southern Russia had com- 

 munication round the Urals with Arctic seas, so that we should 

 expect to find Arctic forms in the Mors deposits, both Arctic 

 forms and forms from the warmer southern seas in the Simbirsk 

 deposits and sub-tropical ones in the Barbados and Oamaru earths. 

 And so it seems to be. This hypothesis also seems to indicate 

 that diatoms are not of very early origin, which suggestion seems 

 to be confirmed by the fact that none have so far been found in 

 the coal measures. Diatom systematics are usually supposed to 

 be overcrowded and confused by the creation of many species and 

 varieties based often on very small difierences. The cause of this 

 confusion lies not with the classifier, but with the diatoms. Their 

 skeletons are indestructible, so that not only species and varieties, 

 but intermediate forms are preserved, and as Evolutionists we 

 should be very grateful for this. It gives us an opportunity of 

 studying continuity and discontinuity in nature. Sir Nicolas 

 then proceeded to define the two terms with examples. Varia- 

 tion is said to be continuous when the differences between two 

 states of the variable are infinitely small, and discontinuous when 

 these differences are not infinitely small, but finite, small or large. 

 A photograph was then shown on the screen of 119 forms of the 

 genus Coscinodiscus. The genus contains about 300 species, and 

 an infinite number of intermediate forms very difficult to classify. 

 Some of the forms very nearly approach continuity, while others 

 represent real or it may be only quasi-discontinuity — real if inter- 

 mediate forms do not exist, quasi if for some reason, as on the slide 

 shown, they do not appear. The changes of absolute continuity 

 are capable of neither numeration, measuiement nor observation, 

 and so we observe that many changes in nature appear as a 

 so-called " jump." After further remarks and examples of dis- 

 continuity in nature. Sir Nicolas returned to the deposits. They 

 show several interesting features in common : (1) Each has a 

 dominating form or group of forms — in Simbirsk, Coscinodiscus 

 symholophorus, Actinoptychus heterostrophus and the Triceratium- 

 Hemiaulus group ; in Mors, the Trinacria-Hemiaulus group ; 

 in Oamaru, Stephanopyxis Grunowii and the Triceratium-Trinacria 

 group, the latter dominating in species and varieties though not 



