INTRODUCTION 



" Jedes Thier erscheint als eine Summe vitaler Einheiten, von denen jede den vollen 

 Charakter des Lebens an sich tragt." VlRCHOW. 1 



DURING the half-century that has elapsed since the enunciation of 

 the cell-theory by Schleiden and Schwann, in 1838-39, 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 structure of plants and animals under 

 one point of view by revealing their common plan of organization. It 

 was through the cell-theory that Kolliker and Remak opened the way 

 to an understanding of the nature of embryological development, and 

 the law of genetic continuity lying at the basis of inheritance. It 

 was the cell-theory again which, in the hands of 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 expression of cell-activi- 

 ties. And at a still later day it was through the cell-theory that Hert- 

 wig, 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 phe- 

 nomena under a common point of view or has accomplished 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. 



And yet the historian of latter-day biology cannot fail to be struck 

 with the fact that these two great generalizations, nearly related as 

 they are, have been developed along widely different lines of research, 

 and have only within a very recent period met upon a common ground. 

 The theory of evolution originally grew out of the study of natural 

 history, and it took definite shape long before the ultimate structure 

 of living bodies was in any degree comprehended. The evolutionists 



1 Cellular pathologic, p. 12, 1858. 



B I 



