326 SCIENCE PROGRESS 



snail. The observation is of no value as a comparison of 

 functional capability in the two types of cell until we are 

 assured that in the two cases the conditions of experiment 

 were alike. But the normal tissue fluids of the two animals 

 contain different percentages of inorganic salts, which may well 

 have been the factors essential to the differences observed. The 

 comparison cannot therefore be made strictly until both cells 

 have been brought into a condition of equilibrium in similar 

 fluids. If this is to be done the conditions must be artificial 

 for one cell or for both. 



This use of artificial conditions is at once the essential 

 method of investigation, and the chief source of difficulty which 

 the inquiry presents. In the first place we cannot deny that 

 the need for testing different cells under uniform conditions 

 will often present grave obstacles from the experimental side. 

 As long as comparison is being made between the different cells 

 of a single species there will not be much to fear ; but when 

 the comparison is between the cells of animals widely remote 

 in phylogenetic relationship, the normal conditions may be so 

 far different that it will be scarcely possible to find a single 

 set of conditions under which both types of cell will survive. 

 As a particular example of the sort of difficulty which may be 

 encountered from this quarter, one thinks of the wide difference 

 of composition which the tissue fluids of different animals 

 present. The concentration of the blood of marine invertebrates 

 and of the lower fishes is about equal to that of sea-water ; that 

 of animals living in fresh water is often no more than one-fifth 

 of this. Recent physiological research shows, indeed, that it is 

 possible to bring cells accustomed to one concentration of tissue 

 fluid into steady equilibrium with a widely different concen- 

 tration in a time not greater than a few hours. But the necessity 

 of such a process obviously adds much to the difficulty of 

 comparing cells taken from different animals. 



Quite apart from such experimental difficulties, the use of 

 artificial conditions will undoubtedly offend that prejudice which 

 the human mind so often shows against any investigation of the 

 phenomena of life under conditions other than those normal to 

 the organism. The basis of this objection seems to be the idea 

 that, so long as we investigate and describe the normal sequence 

 of biological phenomena, we are doing all that can possibly be 

 done towards the scientific explanation of life, whereas to in 



