OF THE GENERA NAVICULA AND CYMBELLA. 409 



microcephala, wliicli I will also describe. If we go down or up 

 this ladder we shall see how insensibly the intermediate forms 

 pass from one into another. 



Now before I proceed along this ladder I should like to state 

 that the majority of freshwater Diatoms as a rule favour cold 

 climates, cold waters, cold mountainous streams. It has been 

 even ascertained that the higher the altitudes they are found in, 

 the more they thrive and the more pronounced and dense are 

 their markings. It would seem therefore natural to suppose 

 that the parental fossil form I am speaking about, Navicular 

 Monmouthiana, appeared on the scene in the State of Maine, at 

 the latitude of 45 degrees north, in other words at the latitude of 

 the Great Lakes of Canada, about the end of the Pliocene period, 

 just about the beginning of the Glacial epochs. At that time, 

 as is admitted by geologists, the continent which connected North 

 America with Europe — the so-called Atlantis — under the influence 

 of volcanic action began to break up, opening into more southern 

 seas a passage for the colder waters of the Polar regions, and 

 bringing down the great glacial deposits of ice as far south as 

 the Great Lakes. At that time the climatic conditions on our 

 earth began to undergo great and important changes : until then 

 the uniformly tropical climates prevailing much farther north 

 than the 45th parallel were beginning to cool and to give way to 

 colder conditions, heralding in the Glacial epochs. It was at the 

 end of the Pliocene that the large proboscidian mammals began 

 also to disappear from high latitudes, or at least that some of 

 them began, like the mammoths, to have fur coats. It was after 

 the end of the Glacial epochs, and with the advent on our planet 

 of temperate climates, that there began also the distribution of 

 the Angiosperm Flora, resulting in the differentiation of the many 

 groups of modern vegetation. It would therefore be natural to 

 suppose that about the end of the Glacial periods the parental 

 form, Navicula MonmoutJiiana, started upon its path of variations, 

 evolving the many descendant forms I am about to describe. If 

 we admit that this work of evolution started on its way after the 

 disappearance from the regions of the Great Canadian Lakes of 

 the last ice sheet of the Glacial epoch, we could reasonably assign 

 for it the whole of the Quarternary period up to the present day. 

 In the light of the comparison with higher mathematics with 

 which I began my paper, we may suppose Navicula Monmouth- 



