In pursuance of this question we have found that the complicated machines have been built out of the simpler ones by the action of known forces and laws.  The factors in this machine building are simply those of the fundamental vital properties of the simplest protoplasmic machine.  Reproduction, heredity, and variation, acting under the ever-changing conditions of the earth’s surface, are apparently all that are needed to explain the building of the complex machines out of the simpler ones.  Nature has forces adequate to the building of machines as well as forces adequate to the formation of chemical compounds and worlds.

But here again we are unable to base our explanation upon chemical and physical forces.  Reproduction, heredity, and variation are properties of the cell machine, and we are therefore thrown back upon the necessity of explaining the origin of this machine.  Can we find a mechanical or chemical explanation of the origin of protoplasm?  A chemical explanation of the cell is impossible, since it is not a chemical compound, but a piece of mechanism.  The explanation given for the origin of animals and plants is also here apparently impossible.  The factors upon which that explanation depended are factors of this completed machine itself, and can not be used to explain its origin.  We are left at present therefore without any foundation for further advance.  The cells must have had a history of construction, but we do not as yet conceive any forces which may be looked upon as contributing to that history.  Whether life phenomena can be manifested by any mixture of compounds simpler than the cell we do not yet know.

The great problems still remaining for solution, which have hardly been touched by modern biology in all its endeavours to find a mechanical explanation of the living machine, are, therefore, three.  First, the relation of mentality to the general phenomena of the correlation of force; second, the intelligible understanding of the mechanism of protoplasm which enables it to guide the blind chemical and physical forces of nature so as to produce definite results; third, the kind of forces which may have contributed to the origin of that simplest living machine upon whose activities all vital phenomena rest—­the living cell.

INDEX.

A.

Absorption of food, 20.

Acquired characters, inheritance of, 164, 165, 166, 167, 171. —­variations, 159, 160.

Amoeba, 73.

Anatomical evidence for evolution, 142.

Aquacity, 80.

Arm compared with wing, 144.

Aristotle, 1.

Assimilation, 80, 124, 149, 176.

Asters of dividing cells, 98.

B.

Barry, 63, 64.

Bathybias, 84.

Biology a new science, 1, 5, 15.

Blood, 35, 36, 38, 69, 73.