30 THE BIOLOGICAL PROBLEM OF TO-DAY 



quite independent of the theory of cells, because, 

 while cells are important units in morphological 

 structure, independently of this they cannot be 

 regarded as important units. ' By a unit,' he 

 insists, ' we must understand, in a physical sense, 

 a system of material particles. In the organic 

 world there are very many kinds of higher and 

 lower units; vegetable and animal individuals, 

 organs, tissues, groups of cells (in the vegetable 

 kingdom, for instance, vessels and sieve- tubes), 

 cells, parts of cells (plant cell-membranes, plasma, 

 granules, and crystalloids, starch - grains, fat- 

 globules, and so forth), micelke, molecules, atoms. 

 In morphology and physiology, sometimes one kind 

 of unit, sometimes another, comes characteristically 

 and notably into evidence. That being so, there is 

 no reason why a special kind of unit should be 

 exalted in a general theory.' 



Athough, with Naegeli, we must recognise and 

 keep in view the presence of a large number of 

 higher and lower units in the organic world, a fact 

 upon which I shall lay considerable emphasis later, 

 we must none the less recognise that, among all 

 elementary units, cells are most the conspicuous, 

 morphologically and physiologically, in the whole 

 organic realm. In actual research this is avowed 

 very practically, as a glance at the biological litera- 

 ture of the last thirty years will show. Especially 

 in the study of heredity, the cell is a unit that 

 cannot be neglected, for it has been established that 

 spores, ova, and spermatozoa, the units by which 

 species are preserved in reproduction, both in the 



