essential to the earlier machine might, although useless, be found in the new, provided it did not interfere with efficiency.

Then he may also point out that the development was not invariably towards higher specialisation, for, as in the case of the steam-hammer, the new machine was sometimes less highly specialised than its

antecessor.

All these features in the development of the steam-engine have parallels in the evolution of life.

Like the first steam-engine, the earliest forms of life were the simplest. Embryology tells us that, so far as the organisations were alike, every new type of life was based on its antecessor, and that some organs of an antecessor became transformed in its successor; while occasionally the fragments of an organ, essential in the old type, are to be found, although apparently useless, in the new.

Again, the expert might dwell on the wide gap in some cases between an old type of engine and a new, in whose structure intermediate developments had been entirely superseded by new inventions.

Similarly, and possibly for similar reasons, wide gaps in the evolution of life some

times occur between a new type and its immediate (known) antecessor.

Comparing engines of the same variety, the expert might observe that no two engines are absolutely identical, but differ in respect of quality of materials or of workmanship, or in efficiency, just as from analogous causes two animals of the same race differ in expression of type or in energy.

Reviewing the development as a whole, the expert might explain how some of the earlier types of engine have, like certain types of organisms, survived, while others, intermediate between them and the engine of to-day, have disappeared.

The development of the steam-engine progressed in accordance with man's necessities. Many of our present engines would have been useless until the necessity for them arose or conditions necessary for their use were provided, and if the scheme of evolution were fully disclosed, we should probably understand the reason for the successive developments of life-why some types persist through ages unchanged, and why others disappeared, or became more or less highly specialised.

Are all these parallels between the evolu

tion of life and of the works of man mere coincidences?

Do they not indicate that the intelligence. and power manifested in the development of the steam-engine are, though infinitely lower in degree, akin to those that evolved successive forms of life, and do they not justify the presumption, that there is a certain analogy between man's method of developing his works and the evolution of life?

If, then, we are to inquire into Nature's methods, should we not advance most surely by comparison, based on our observation and experience, of the processes of Nature and of man?

The homology throughout Nature suggests that the laws of the forces which we call life are analogous to those of the forces that pervade inanimate Nature, and are we not bound to assume that in the evolution of life there was no effect without a corresponding Cause, nothing spontaneous or accidental?

CHAPTER IX.

THEORY OF EVOLUTION.

WE now proceed to formulate our Theory of Evolution, recognising fully that it shall be subject to Huxley's test, that "Every hypothesis is bound to explain, or at anyrate not be inconsistent with, the whole of the facts which it professes to account for, and if there is a single one of these facts which can be shown to be inconsistent with (I do not merely mean inexplicable by, but contrary to) the hypothesis, the hypothesis falls to the ground-it is worth nothing."

1

The evolution of life was by successive steps-each step a distinct new type.

The first forms of life (which all theories assume) consisted of a simple cell, that by inherent force (which we call "life-force") multiplied by producing other similar cells. 1 Darwiniana, p. 463.

Thereafter more complex life-forces, owing existence to the same constructive Power as the first, successively evolved new organisms, widely diversified in form, and, as a rule, more and more highly specialised, until the evolution culminated in Man.

In the process of evolution the Specific life-force of an existing organism was utilised in evolving a new type, so far as the old and the new were alike: in other words, the new structure was built on old foundations.

Evolution was not invariably towards higher specialisation, nor was it restricted to one line of development. Like a lofty, wide-spreading tree, the main stem of evolution put forth branches in all directions, some more or less divergent from the upright trunk, others horizontal, and not a few downwards; but the evolution throughout was on the same principles-development by utilising, as far as serviceable, the Specific life-force of an existing type to evolve a new, and, growth by accretion of cells.

The differentiation between successive types was effected by modifying, or adding to, an existing Specific life-force.

Thus not only every species, but every