competent to perform all functions, and one and the same portion of
protoplasm may successfully take on the function of feeding, moving, or
reproducing apparatus. In the highest, on the contrary, a great number
of parts combine to perform each function, each part doing its allotted
share of the work with great accuracy and efficiency, but being useless
for any other purpose.

On the other hand, notwithstanding all the fundamental resemblances
which exist between the powers of the protoplasm in plants and in
animals, they present a striking difference (to which I shall advert
more at length presently), in the fact that plants can manufacture fresh
protoplasm out of mineral compounds, whereas animals are obliged to
procure it ready made, and hence, in the long run, depend upon plants.
Upon what condition this difference in the powers of the two great
divisions of the world of life depends, nothing is at present known.

With such qualifications as arise out of the last-mentioned fact, it may
be truly said that the acts of all living things are fundamentally one.
Is any such unity predicable of their forms? Let us seek in easily
verified facts for a reply to this question. If a drop of blood be drawn
by pricking one's finger, and viewed with proper precautions, and under
a sufficiently high microscopic power, there will be seen, among the
innumerable multitude of little, circular, discoidal bodies, or
corpuscles, which float in it and give it its colour, a comparatively
small number of colourless corpuscles, of somewhat larger size and very
irregular shape. If the drop of blood be kept at the temperature of the
body, these colourless corpuscles will be seen to exhibit a marvellous
activity, changing their forms with great rapidity, drawing in and
thrusting out prolongations of their substance, and creeping about as if
they were independent organisms.