6 PHYSIOLOGY 



being, only such would survive in which the process of exothermic 

 disintegration tended towards a condition of greater stability, so that 

 the process might come to an end, and the organism or compound be 

 enabled to await the more favourable conditions necessary for the 

 continuance of its growth. With the continued cooling of the earth, 

 the new production of endothermic compounds would become rarer 

 and rarer ; and in all probability the beginning of life, as we know it, 

 was the formation of some complex substance, analogous to the present 

 chlorophyll corpuscles, with the power of absorbing the newly pene- 

 trating sun's rays and utilising them for the endothermic formation of 

 further unstable compounds. Once given an unstable system, such as 

 we have imagined, the great principle laid down by Darwin, viz. 

 survival of the fittest, will suffice to account for the production from 

 it by evolution of the ever-increasing variety of living beings which 

 have appeared in the later history of this globe. The ' adaptation,' 

 i.e. the reactions of the primitive living material to changes in its 

 environment, must become ever more and more complex, since only 

 by means of increasing variety of reaction is it possible to provide for 

 the stability of the system within greater and greater range of external 

 conditions. The difference between higher and lower forms is there- 

 fore one of complexity of reaction, or of range of adaptation. 



In all the physiological processes which we shall study in the course 

 of this work, adaptation will be found the constant and guiding 

 quality. The naked protoplasm of the plasmodium of Myxomycetes, if 

 placed on a piece of wet blotting-paper, will crawl towards an infusion 

 of dead leaves, or away from a solution of quinine. It is the same 

 property of adaptation, the deciding factor in the struggle for existence, 

 which impels the greatest thinkers of our time to spend long years of 

 toil in the invention of the means for the offence and defence of their 

 community, or for the protection of mankind against disease and 

 death. The same law which determines the downward growth of the 

 root in plants is responsible for the existence to-day of all the sciences 

 of which mankind is proud. 



This " adjustment of internal to external relations " is possible, 

 however, only within strictly defined limits, limits which increase in 

 extent with rise in the type of organism, and in the complexity of its 

 powers of reaction. Some of these limiting conditions we shall have 

 to study in the next chapter. Among the chief of them are tempera- 

 ture, and the presence of food material and of oxygen. At the present 

 time the limits of temperature may be placed between and 50 C. 

 Many organisms, however, are killed by the alteration of only a few 

 degrees in the temperature of their environment. Every shifting of a 

 cold or warm current in the Atlantic, in consequence of storms on the 

 surface, leads to the destruction of myriads of fish and other denizens 



