3 2 4 SCIENCE PROGRESS 



of these problems would be of real assistance towards the 

 understanding of the history of animal evolution. 



Let us consider first the tracing out of those evolutionary 

 changes which have been characterised as changes of functional 

 capability. Is the knowledge of such changes really essential 

 to the understanding of the history of evolution ? In other 

 words, has the evolution of functional capability been a factor 

 of the first importance in the differentiation of organisms ? We 

 may suppose for a moment that it has not, in order to discover 

 where such a supposition will lead. If the supposition be true, 

 then the only physiological difference between the various parts 

 of an organism lies in the functional opportunity which the 

 various cells encounter. To take a single example : the heart 

 muscle and the limb muscle of the frog behave differently only 

 because they are subjected to different conditions and stimuli 

 in the organism. The absurdity of this inference is at once 

 manifest. Every student of elementary physiology has made 

 the experiment of observing the behaviour of a limb muscle 

 and of heart muscle when placed under like conditions, namely, 

 when excised and immersed in Ringer's fluid. This simple 

 experiment in functional capability shows the heart muscle 

 beating rhythmically, the limb muscle quiescent. There can 

 be no doubt that the two sorts of muscle are endowed with 

 properties inherently different. In fact it is almost absurd to 

 raise the question. The whole selective action of drugs is 

 dependent on the differentiation of functional capability. The 

 assumption that such differences are of fundamental importance 

 lies at the very root of modern physiological research. 



Though we may recognise clearly enough that the cells 

 of an organism do differ in functional capability, and that such 

 differences have been potent factors in the evolution of the 

 organism, yet our knowledge of those differences is wanting in 

 precision. Only in the case of very few types of cells— practi- 

 cally only of those types which we group together as excitable 

 cells — are we beginning to get any clear definition of the 

 existing differences of functional capability ; while as to the 

 evolutionary history of the differentiation we know scarcely 

 more than nothing. There is no need to look for such remote 

 problems as the past functional history of those cells of the 

 suprarenal body whose secretion is so essential to the working 

 of the mammalian organism. Even in such a simple and well- 



