I 8 COLLEGE ZOOLOGY 



from the following quotation from E. B. Wilson, the foremost 

 investigator of cellular phenomena in this country. 



' During the half-century that has elapsed since the enuncia- 

 tion of the cell-theory by Schleiden and Schwann, in 1838-1839, 

 it has become ever more clearly apparent that the key to all 

 ultimate biological problems must, in the last analysis, be sought 

 in the cell. It was the cell-theory that first brought the struc- 

 ture of plants and animals under one point of view, by revealing 

 their common plan of organization. It was through the cell- 

 theory that Kolliker, Remak, Nageli, and Hofmeister opened 

 the way to an understanding of the nature of embryological de- 

 velopment, and the law of genetic continuity lying at the basis 

 of inheritance. It was the cell-theory again which, in the hands 

 of Goodsir, Virchow, and Max Schultze, inaugurated a new era 

 in the history of physiology and pathology, by showing that all 

 the various functions of the body in health and in disease are 

 but the outward expressions of cell activities. And at a still 

 later day it was through the cell-theory that Hertwig, Fol, Van 

 Beneden, and Strasburger solved the long-standing riddle of 

 the fertilization of the egg and the mechanism of hereditary 

 transmission. No other biological generalization, save only the 

 theory of organic evolution, has brought so many apparently 

 diverse phenomena under a common point of view, or has accom- 

 plished more for the unification of knowledge. The cell-theory 

 must therefore be placed beside the evolution-theory as one of the 

 foundation stones of modern biology." 



6. PLANTS CONTRASTED WITH ANIMALS 



It is easy to choose characteristics that will serve to distinguish 

 a tree from a man, but the separation of the simplest animals 

 from the simplest plants is a more difficult problem. In fact, 

 there are at the present time a number of organisms that are 

 claimed by both botanists and zoologists. There is no single 

 peculiarity which can be used in all cases to discriminate between 



