4 INTRODUCTION 



vital functions are performed by a single cell ; in the higher types they 

 are distributed by a physiological division of labour among different 

 groups of cells specially devoted to the performance of specific 

 functions. The cell is therefore not only a unit of structure, but 

 also a unit of function. " It is the cell to which the consideration 

 of every bodily function sooner or later drives us. In the muscle- 

 cell lies the riddle of the heart-beat, or of muscular contraction ; in the 

 gland-cell are the causes of secretion ; in the epithelial cell, in the 

 white blood-cell, lies the problem of the absorption of food, and 

 the secrets of the mind are slumbering in the ganglion-cell. ... If 

 then physiology is not to rest content with the mere extension of our 



■;-.v-..N 







• -■.;-■:■-■*«- ■• '-.{1 ;-i f ■• c (""n ^i^'To '.V ~ 



C V 



Fig. 2. — Amxba Proteus, an animal consisting of a single naked cell, X 280. (From Sedgwick 

 and Wilson's Biology.) 



n. The nucleus ; w.v. Water-vacuoles ; c.v. Contractile vacuole ; f.v. Food-vacuole. 



knowledge regarding the more obvious operations of the human 

 body, if it would seek a real explanation of the fundamental phe- 

 nomena of life, it can only attain its end through the study of ccll- 

 pJiysiologyy ^ 



Great as was the impulse which the cell-theory gave to anatomical 

 and physiological investigation, it did not for many years measurably 

 affect the more speculative side of biological inquiry. The Origin of 

 Species, published in 1859, scarcely mentions it; nor, if we except 

 the theory of pangenesis, did Darwin attempt at any later period to 

 bring it into any very definite relation to his views. The cell-theory 

 first came in contact with the evolution-theory nearly twenty years 



^ Verworn, AUgeiiteine Physiologic, p. 53, 1895. 



