328 SCIENCE PROGRESS 



study will be the type of cell in which that function is most 

 obvious and most highly elaborated. Never shall we penetrate 

 de novo into the complex of functional capabilities which a 

 unicellular organism presents, select one capability and trace 

 it upward until it emerges as the specialised property of 

 some group of highly evolved cells. We should fail simply 

 for want of experience and technique gained on the more 

 specialised case. The physiology of the cell must begin, not 

 with that maid-of-all-work the unicellular organism, but with 

 such cells as are as nearly as possible unifunctional. This 

 decision is as much a matter of history as of prophecy. 

 When we speak of the conduction of the excitatory state in 

 unicellular organisms we are speaking of phenomena which 

 were first seen and traced in specialised nerve-cells, were after- 

 wards recognised by similar methods in muscle-cells, and were 

 only later detected in unicellular organisms by investigators 

 who had the previous knowledge to guide them. The only 

 practicable method for the study of any function will be to 

 investigate that function first in some cell in which it appears 

 in a highly elaborated state, and with the help of experience 

 and technique so gained to trace it back through succeeding 

 degrees of less specialisation and greater obscurity. 



Here we encounter a grave difficulty. If we were acquainted 

 with every modification which any one functional capability 

 exhibited throughout the animal kingdom, we should still be 

 unable to arrange the various modifications found in the order 

 of their succession in evolution. We have no sure ground 

 for asserting that it would be just, for example, to say that 

 every function had developed along the line of increasing 

 complexity. The idea presents itself, of course, that we might 

 make use of the phylogenetic succession of species and arrange 

 the functions observed in the order of the species from which 

 they were taken. This might be a practicable method, if we 

 were so fortunate as to have at our command a number of 

 animals which formed a continuous series of successive steps 

 in a process of evolution. As a fact, it is not to be expected 

 that the animals living at any one time will ever form such a 

 series. The animal kingdom is made up of a number of species, 

 each of which represents the end of one line of a branching 

 system. The condition of things which we are seeking would 

 demand that a species, whenever it had by variation produced 



