148 



THE DIATOMS OF THE SEDBERGH DISTRICT. 



A STUDY IN EVOLUTION.* 



R. H. PHILIP. 



(plate VI.). 

 Seven years ago, at a previous meeting of the Yorkshire 

 Naturahsts' Union at Sedbergh, the writer made a collection of 

 the Diatoms of the district- — a collection which bears a certain 

 special interest in its relation to a new theory of the evolution 

 of the Diatomaceae recently propounded by the French Dia- 

 tomist, M. Peragallo. By all authorities great importance has 

 been attached to a rift or slit which passes down the centre of 

 the valve from one extremity to the other, in certain genera, 

 and known as the raphe. It is universally agreed that this rift 

 is inseparably connected with the mysterious power of appar- 

 ently voluntary motion, and though the authorities are bj^ no 

 means agreed as to the method by which this motion is effected, 

 there can be no doubt that without a raphe no motion is possible. 

 Hence the Diatomacese have been classified in three sub- 

 families : the Raphidieje, having a true raphe on at least one 

 of the two valves ; the Pseudo-Raphidieae, having a blank 

 space simulating a raphe on at least one valve ; and the Crypto- 

 Raphidieae — or, as M. Peragallo more accurate^ prefers to call 

 them, the Ana-Raphidieae, which possess neither a true raphe, 

 nor the appearance of one on either valve. The first division 

 (with some exceptions to be referred to later) are self-motile. 

 The other two divisions are incapable of self-movement. 



M. Peragallo holds that the origin from which the evolution 

 of Diatoms started was from the marine Radiolaria, an ahiaeb- 

 oid organism having a siliceous skeleton, and consisting for 

 the rest only of a protoplasmic jelly with the usual nucleus and 

 nucleolus, and generally considered to be a member of the 

 animal kingdom. Now, as the same authority unhesitatingly 

 accepts the Diatoms as true vegetables, it may cause us some 

 sin-prise to be asked to believe that they can have been evolved 

 from an animal ancestor. Perhaps a more correct way of 

 putting it would be to regard Radiolaria and Diatoms alike 

 as descended from a common ancestor in which the differentia- 

 tion of animal and vegetable had not been attained. However 

 this may be, it will be sufficient for us to note that those forms 



* Abstract of a Paper read at tlis meeting of the Yorkshire NaturalLsts' 

 Union at Sedberg-h, August 1909. 



Naturalist, 



