CLASSIFICATION OF THE ANIMAL KINGDOM. 33 
This, too, is primarily a molecular effect. If the works of 
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 differences 
to the mechanism. So is it with living beings: the ultimate 
molecular mechanism is unknown to us. 
Could we but render these molecular movements sistle 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 unification of knowledge toward which science 
is tending. But it also follows that without complete knowl- 
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