6 INTR ODl'C TION 



parison is not less suggestive to the physiologist than to the mor- 

 phologist. In the one-celled forms all of the vital functions are 

 performed by a single cell. In the multicellular forms, on the other 

 hand, these functions are not equally performed by all the cells, but 

 are in varying degree distributed among them, the cells thus falling 

 into physiological groups or tissues, each of which is especially de- 

 voted to the performance of a specific function. Thus arises the 

 " physiological division of labour" through which alone the highest 

 development of vital activity becomes pos.sible ; and thus the cell 

 becomes a unit, not merely of structure, but also of function. Each 

 bodily function, and even the Hfe of the organism as a whole, may 

 thus in one sense be regarded as a resultant arising through the inte- 

 gration of a vast number of cell-activities ; and it cannot be adequately 

 investigated without the study of the individual cell-activities that lie 

 at its root.^ 



The foregoing conceptions, founded by Schwann, and skilfully 

 developed by Kolliker, Siebold, Virchow, and Haeckel, gave an im- 

 pulse to anatomical and physiological investigation the force of which 

 could hardly be overestimated; yet they 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, 

 with the important exception of the theory of pangenesis, did Darwin 

 attempt at any later period to bring it into any very definite relation 

 to his views. The initial impulse to the investigations that brought 

 the cell-theory into definite contact with the evolution-theory was 

 given nearly twenty years after the Origin of Species, by researches 

 on the early history of the germ-cells and the fertilization of the 

 ovum. Begun in 1873-74 by Auerbach, Fol, and Biitschh, and 

 eagerly followed up by Oscar Hertwig, Van Beneden, Strasburger, 

 and a host of later workers, these investigations raised wholly new 

 questions regarding the mechanism of development and the role of 

 the cell in hereditary transmission. Through them it became for the 

 first time clearly apparent that the general problems of embryology, 

 heredity, and evolution are indissolubly bound up with those of cell- 

 structure, and can only be fully apprehended in the light of cytologi- 

 cal research. As the most significant step in this direction, we may 

 regard the identification of the cell-niiclcus as the vehicle of inheri- 



1 Cf. pp. 58-61. " It is to the cell that the study of every bodily function sooner or later 

 drives us. In the muscle-cell lies the problem of the heart-beat and that of muscular con- 

 traction ; in the gland-cell reside 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 

 hidden in the ganglion-cell. ... If then physiology is not to rest content with the 

 mere extension of our knowledge regarding the gross activities of the human body, if it 

 would seek a real explanation of the fundamental phenomena of life, it can only attain its 

 end through the study ol cell-physiology''' (Verworn, Alkyweine Physiologie, p. 53, 1895). 



