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Thus it has become clear that the elements are all related to one another. It is not known how to explain this relationship—perhaps they have been evolved in an orderly manner from something else—but, at all events, matter is not only indestructible (Lavoisier), but it makes up a unitary system. To-day we feel sure that we are acquainted with nearly all the stable varieties of matter that exist in the universe, though of course there remain a great variety of arrangements of this matter which are unknown to us.

3. Biological Evolution

The only well-known phenomenon that cannot be completely described in terms of matter and energy is life, with its peculiar characteristics of consciousness and thought. In the year 1859 biology yielded to the unifying idea of Charles Darwin. Many had previously suspected that all living things are blood relations; the discoveries of embryologists in particular had proved that the similarities among living things are far more profound than had been formerly realized. But Darwin provided a plausible explanation of the development of more complex beings by a continuous evolutionary process, and this led to the world's final decision in favor of the hypothesis of transformation.

It is possible that some of Darwin's hypotheses may in the end be discarded, but it appears to be wholly unlikely that the world will ever give up its belief in the evolution of organic beings, in all their multitudinous forms, from earlier and simpler types, and probably originally from one or more exceedingly simple forms.

Finally, the change in the relation of science to civilization, accomplished in the nineteenth century, marks a new epoch in history.