i8 PHYSIOLOGY OF THE INVERTEBRATA. 



and ketones, aids us considerably in understanding the origin 

 of various secretory products found in the Invertebrata as well 

 as in the Vertebrata. 



Living protoplasm is a substance which is constantly 

 undergoing chemical changes. It is the chemical and' 

 physical properties of this complex substance, diversely'l 

 modified, which underlie all the vital functions — nutrition,, 

 secretion, growth, reproduction, motility, &c. Of these 

 functions the most important is nutrition, the double and 

 perpetual movement of molecular renovation of the living 

 substance. Without nutrition there can be no growth, no 

 reproduction, no moveijient, and in fact no physiological 

 function whatsoever. It has been stated that " life can be 

 conceived of as reduced to its most simple expression, to 

 mere nutrition. A being capable of nourishing itself, and' 

 destitute of every other property or function, which , after all,, 

 is only a simple extension of the nutritive property, its life- 

 will be only an individual life;" a time will come when the 

 nutritive functions will have less energy — then " the nutritive- 

 residue, incompletely expulsed, will impregnate the living 

 tissues and liquids obstructing them." Such obstruction 

 necessarily interferes with physiological activity, and ulti- 

 mately ends in complete arrest. "When this stage arrives,. 

 the organism, no longer capable of adjusting its " internal 

 relations to external relations,"* undergoes those chemico-- 

 biological changes which finally result in its molecules (as 

 new combinations) once more re-entering the mineral king- 

 dom — or the world of inanimation. On the other hand, if 

 the nutritive activity of a living organism "is sufficiently 

 energetic to rise, as it were, to excess, even to growth and 

 reproduction, the being is sure of living in its offspring ; it 

 fills its place in the innumerable crowd of living beings, and 

 can even, according to the doctrine of evolution, become 

 the source of a superior organised type, can ascend in the- 

 hierarchy of life." 



* Mr. Herbert Spencer's definition of " life." 



