be entirely different from any of those that inhabit this earth. The plan of structure of organic forms depends entirely upon the initiative which first launched each type upon its career. This initiative is wholly fortuitous. The vertebrate type of animals, for example, must be looked upon as due to some primordial accident, as it were, i. e., some coincidence of causes, external and internal, at the appropriate time and place, that happened to determine that type of structure which proved better adapted to sustain the highest organization thus far attained in the animal kingdom. If this particular type had not chanced to be tried, some other would have stood highest, but it is as likely to have been a still better one as to have been a poorer one for the purpose. If the planet Mars is really the home of living beings the chances of the vertebrate type of structure occurring there are only as one to infinity. Yet some superior type may be developed there. And if there be on that planet or anywhere else in the solar system or in the universe a master being related to other beings in any such way as man is related to the other living creatures on this earth, the chances are again infinity to one against his possessing the form or any of the leading physical attributes of human beings.
All this may at first sight look like wild Utopian speculation. But its utility does not lie in any knowledge it yields as to the inhabitants of other planets. It lies in teaching the great lesson that no knowledge of anything can be gained by speculation, and that our only knowledge consists in the actual investigation of facts that lie within our reach. We must study the tangible, visible, demonstrable world and find out what it contains. There is no telling what we shall find. No preconceived notions of what we ought to find, much less of what we ought not to find, must influence the quest for truth. This is not, however, to discourage the use of hypotheses. They are the searchlights of science. But their use requires due caution, and a hypothesis must not be confounded with a thesis.
Now, while it is true that all those aggregations of cosmic elements that give multiplicity and variety to the content of the uni-
