150 Philip : The Diatoms of the Sedhergh District. 



estuaries, shaking them up with the mud, and then placing theni 

 on a dish in the sunhght. In a short space of time they will 

 have worked their way up through the mud, and covered its 

 surface with a brown film. It is not with them a question of 

 journeying anywhere, of pursuing prey, or of escaping from 

 enemies, it is simply just sufficient to enable them to preserve 

 their freedom. This limited power of motion has evidently 

 been an important advantage in the struggle for existence, 

 since we find that the Raphidieae are much the largest of the 

 sub-families to-day. Taking for instance the list of 600 species 

 and varieties in the " Diatoms of the Hull District," we find 

 that about two-thirds are Raphidieae and less than a sixth 

 Ana-Raphidiese. Figs. 4, 5, and 6 on Plate VII. are typical 

 examples of motile Raphidieae. 



Established on the coast, the Raphidieae increased in num- 

 bers, and began to find their way up the estuaries and rivers 

 into the internal waters of the land, and in marshes, lakes, 

 ponds, streams and springs evolved into different forms suited 

 to their respective environment. 



And now we come to the Diatoms of the Sedbergh district. 

 The waters of this neighbourhood are ghylls, little streams 

 cutting their way through deep gorges on the hill sides, at all 

 times rapid in flow, and in rainy seasons liable to become violent 

 torrents. In such waters, how are our tiny organisms to be 

 able to maintain themselves ? The Ana-Raphidieae, with their 

 circular or polygonal forms and helpless immobility, would be 

 swept into crevices and stifled in mud, if they had ever got up 

 here — which they never did, for not a single member of the sub- 

 family is to be found here. The Raphidieae, with their limited 

 power of motion, sufficient only to rescue them from premature 

 burial, would scarcely fare better, being liable to be continually 

 carried down to the sea by the rapid waters. Hence new 

 methods must be found if they are to continue to inhabit these 

 streams, and the problem of existence has been solved by 

 several distinct methods. Some of the Raphidieae, of which 

 the Gomphonemas are an example, acquired a stipe, or stalk 

 which anchored them to a rock or water plant, enabling the 

 diatom to float securely in the most turbulent streams ; others 

 like Cocconeis attached themselves by one surface to mosses or 

 plant stems, while others, like Achnanthes, improved on this 

 by piling themselves on one another. Biit now comes the 

 Nemesis of over secimty, they lose their freedom. No doubt 



Naturalist, 



