158 THE EVOLUTION OF MAN. 



case of the single cells is by the absorption by their soft 

 semi-fluid cell-substance of food-material from the sur- 

 rounding fluid; less frequently solid particles are pressed 

 into the cell-substance. Similarly, the worn-out material 

 is discharged, usually in a fluid, seldom in a solid form. 



Adaptation, the most important vital function, is 

 directly connected with nutrition, and plays the most im- 

 portant part in the progressive development of the organism. 

 It is, in reality, the most influential cause of every advance 

 and of all perfection of the organism. Adaptation effects 

 all the modifications or variations which organic forms 

 undergo under the influence of the external conditions of 

 existence ; it is the true cause of every modification. As 

 I have very fully discussed the importance of modification 

 and the various laws of Adaptation in my Generelle 

 Morphologie, and in the " History of Creation," I may here 

 dispense with any further reference to it. I shall only call 

 attention to the fact, that all these various laws of Adapta- 

 tion can appropriately be brought into the two classes that 

 I have there distinguished ; on the one side indirect, or 

 Potential Adaptation, on the other direct, or Actual Adap- 

 tation. I have shown in my Generelle Morphologie (vol. 

 ii. pp. 193-226), that all these varied and important phe- 

 nomena, if regarded from a physiological point of view, can 

 be reduced to the mechanical function of nutrition, and, 

 indeed, to the elementary conditions of cell-nutrition. 



Just as progressive Adaptation is linked with nutrition, 

 so is conservative Heredity linked with reproduction. This 

 latter activity of the organism may also be referred to the 

 former functions. For radically " reproduction is a form of 

 nutrition and a growth of the organism to a size beyond 



