264 SCIENCE PROGRESS. 



memoir (xvii., 1890), and has been finally confirmed by still 

 more recent researches. 



In the meantime (in mem. vii., 1875, and mem. xiii., 

 1887) a beautifully preserved fossil had been described in 

 great detail under the name of Kaloxylon Hookeri. It was 

 then looked upon as a distinct plant, though its discoverer 

 recognised the similarity of its tissues to those of Lyginoden- 

 dron. More recent investigation has proved that Kaloxylon 

 is nothing else but the root of Lyginodendron (31). 



In 1887 a second species of Hcteranginm, remarkable for 

 the extraordinary development of its secondary phloem, and 

 for the exceptional perfection with which its tissues are pre- 

 served, was described. In this memoir Williamson speaks 

 of Lyginodendron and Hete7 r angiw)i as being "possibly the 

 generalised ancestors of both Ferns and Cycads," a view 

 which, in the light of our present more complete knowledge, 

 appears to come near the truth. 



The proof of the existence of this intermediate group 

 was certainly a discovery of the first magnitude, the impor- 

 tance of which, though ignored in this country, was fully 

 recognised on the Continent. 



It cannot be denied that in this and one or two other 

 instances the great merit of Williamson's work was some- 

 what obscured by the prominence which he gave to certain 

 mistaken ideas as to development. He was always disposed 

 to interpret specimens of the same plant which differed in 

 size and structure, as being necessarily developmental stages 

 of one and the same oroan, whereas such differences are 

 just as often due to the order of the branch or to the level 

 at which the section happens to be taken. Consequently 

 when he found some specimens of a stem with little or no 

 pith, and others with a large one, he thought it necessary to 

 explain how the former developed into the latter, and this 

 led to his theory of the development of the pith from 

 " primitive cells which lurked among the vessels," and to 

 further hypotheses which appeared to other botanists to 

 involve mechanical impossibilities. In the special case of 

 Lyginodendron the matter was further complicated by the 

 fact that roots were not always distinguished from small 



