THE EVOLUTION OF ANIMAL FUNCTION 481 



cells. In functional capability as here defined we have a true 

 physiological property which belongs wholly to the cell in which 

 it is found. 



With this distinction made clear we may return to the main 

 issue and inquire, first, whether the functional capabilities of 

 the cells which constitute an organism may legitimately serve 

 as a guide to the course of evolution which the organism has 

 followed. The real question which this involves is whether 

 functional capability is a property which remains constant 

 over considerable periods of evolution, or whether it is subject 

 to frequent adaptive changes following every alteration of 

 environment and habit. Towards the solution of this problem 

 we might proceed in two different ways. We might inquire 

 directly to what extent likeness of functional capability runs 

 parallel throughout the animal kingdom with the course of 

 phylogenetic development already determined by the usual 

 structural criteria ; or we might consider how far functional 

 capability is determined directly by inheritance, and how far 

 it is the result of subsequent influences to which the cells are 

 subjected in the course of an organism's development. In the 

 present state of our knowledge it is not possible to apply the 

 first of these two methods. We have but little evidence as to 

 the degree of difference which functional capability exhibits 

 throughout the animal kingdom. Such experiments as have 

 been made on the functions of individual cells have not been 

 directed in the rigid manner which the investigation of pure 

 functional capability would demand. We are bound, therefore, 

 for the immediate purpose of the argument to put together such 

 evidence as the second method can afford. 



The problem presents itself in the following form. How far is 

 the functional capability of a cell directly determined by inherit- 

 ance, and how far is it dependent upon the environment and 

 activity of the organism to which the cell belongs ? For the little 

 that we know of the directness of inheritance we have to look 

 to the school of Experimental Morphology, or Entwickelungs- 

 mechanik. This school has attempted, by the isolation of single 

 cells in the very early stages of embryonic development, to deter- 

 mine how far the structural characters built up in the course of 

 development are the outcome of the individual properties of the 

 single cells from which they arise, and how far they result from 

 the interaction of other cells belonging to the same organism. 



