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but a power-engine placed on wheels—not the improved power-engine of the present day, but an unimproved and rudimentary form. We might trace the history of other forms in the same way, until we had found the one source for them all.

In this case there can be no genetic connection; each engine is an independent thing, and the only connection is an ideal one in the minds of the inventors; that is, the idea of the steam-engine has gone through a process of evolution, expansion, and perfection, and most of the steps in the process have been embodied in real engines, so that together they form a manifestation or record of the changes that the idea has undergone.

According to the theory of which Agassiz is the most celebrated advocate, the phenomena of life are to be explained in a somewhat similar way. Recognizing all the facts which seem to indicate the evolution of the animal kingdom, and being himself the discoverer of very many of them, he says that the evolution is simply the evolution of an idea in the mind of the Creator, which idea has been embodied in material form in such a way that it can be traced by the study of the animals that form its expression.

The other theory may also be illustrated by an example: When we compare languages which philologists tell us have descended from one parent tongue, we are attracted by their differences only, and it needs careful study and comparison to understand the similarity of plan which underlies them all; but when their history is traced it is seen that they were originally the same, and have become different as the races using them have become more widely separated, and, coming under new and widely different physical conditions, have diverged in their habits, feelings, thoughts, and associations, and have required different forms of speech to supply their need. Here, unlike the case of the steam-engines, the language has been the same all the time, and, although men have been the means by which the change has been effected, they have not been the intelligent cause, but have been unconsciously acted upon by agencies around them.

According to the theory with which Darwin is identified, although he is not by any means the author, but has simply removed some of the most serious objections, all the different forms of life have been evolved from one source in substantially the same way that languages have originated; as animals become exposed to new conditions, new varieties adapted to these conditions arise, and, as animals thus grow different, the parent unimproved forms are unable to struggle with their more perfect descendants and become extinct, so that the animals which would connect dissimilar forms are no longer in existence.

The evidence necessary for the perfect establishment of either of these theories does not seem to have been obtained as yet, and we can only decide provisionally, according to probabilities; but the discus-