General Introduction 13 



structure, and, as may well be imagined, was in possession 

 of the most advanced knowledge. The labours of Schleiden, 

 Von Mohl, Unger, and Naegeli had been sedulous and pro- 

 tracted, and had led to the recognition of the part played 

 by the cell in the construction of plants, and consequently 

 to the establishment of the cell theory. But though the 

 cell as a unit was attracting attention its parts were not 

 clearly appreciated. By many the importance of the proto- 

 plasm was not grasped, nor its identity as the living part 

 of the cell realized. It was frequently spoken of as merely 

 a part of the cell contents. Nothing was known about 

 its differentiation except the existence of a nucleus, and 

 the fact that in some cases its surfaces are not exactly like 

 its more internal part. The continuity of the proto- 

 plasm between cell and cell had not been discovered, 

 though it was suspected in such cases as the segments of 

 sieve tubes. The differentiation of the tissues had not been 

 very fully studied, though some progress had been made 

 by the investigations of Unger and of Naegeli. The know- 

 ledge of the vascular system was very imperfect and no 

 information of any value was forthcoming as to the tissues 

 of nutrition or secretion. The lines of investigation and 

 research were based chiefly upon the formation and 

 character of the cells of the tissues, and there was con- 

 sequently a lack of proper interpretation of their purposes. 

 Their inter-relations had not come into question, and there- 

 fore the study of anatomy could not have its proper per- 

 spective. A special weakness was the absence of a realization 

 that the differentiation of structure is the outcome, more 

 or less immediate, of physiological division of labour. The 

 influence of the environment on structure, one of the great 

 lessons to be drawn from Darwin's teaching, was still 

 unexamined. At the same time much sound knowledge 

 had been obtained. The researches carried out by Naegeli 

 on the various bodies found in cells and on the membranes 



