34 COMPARATIVE PHYSIOLOGY. 



watches were beyond observation, we should not be able to state 

 exactly how the variations observed in different kinds, or even 

 different individuals of the same kind occurred, though these 

 differences might be of the most marked character, such as any 

 one could recognize. Here once more we refer the differ- 

 ences to the mechanism. So is it with living beings : the ulti- 

 mate molecular mechanism is unknown to us. 



Could we but render these molecular movements visible to 

 our eyes, we should have a revelation of far greater scientific 

 importance than that unfolded by the recent researches into 

 those living forms of extreme minuteness that swarm every- 

 where as dust in a sunbeam, and, as will be learned later, are 

 often the source of deadly disease. Like the movements of the 

 watch, the activities of protoplasm are ceaseless. A watch that 

 will not run is, as such, worthless — it is mere metal — has under- 

 gone an immense degradation in the scale of values ; so proto- 

 plasm is no longer protoplasm when its peculiar molecular 

 movements cease ; it is at once degraded to the rank of dead 

 matter. 



The student may observe that each of the four propositions, 

 embodying the fundamental properties of living matter, stated 

 in the preceding chapter, have been illustrated by the simile of 

 a watch. Such an illustration is necessarily crude, but it helps 

 one to realize the meaning of truths which gather force with 

 each living form studied if regarded aright ; and it is upon the 

 realization of truth that mental growth as well as practical 

 efficiency depends. 



CLASSIFICATION OF THE ANIMAL KINGDOM. 



There are human beings so low in the scale as not to possess 

 such general terms as tree, while they do employ names for dif- 

 ferent kinds of trees. The use of such a word as " tree " im- 

 plies generalization, or the abstraction of a set of qualities from 

 the things in which they reside, and making them the basis for 

 the grouping of a multitude of objects by which we are sur- 

 rounded. Manifestly without such a process knowledge must 

 be very limited, and the world without significance ; while in 

 proportion as generalization may be safely widened, is our 

 progress in the uniflcation of knowledge toward which science 

 is tending. But it also follows that without complete knowl-. 

 edge there can be no perfect classification of objects ; hence, 



