at once granted all the efficiency of a final cause, well chosen and successfully attained. The first step and the most important one, that of the formation of favorable varieties, is rapidly and obscurely passed over; while the second step, which follows simply as the consequence of the first step, and with it involves the force of final causes, receives strong emphasis, and, as the survival of the fittest, becomes the gist of the theory. Even this half of the doctrine covers the facts in the majority of cases quite as well when simply and clearly put in the language of final causes. Plants and animals survive because they are fitted to their conditions. This is the primary fact; the secondary fact is that when they push each other on ground relatively common to them, those most fit win the field.

Take the case, so often referred to, of imitation among insects, an inoffensive species finding protection by conforming to the marks of an offensive species. This one is supposed to approach the other in appearance by a protracted series of slight changes. For this variation no sufficient reason is given. The earlier stages of transition would afford no appreciable advantage, while the chances against a continuous series of variations in the right direction are incredibly great. So far the reasoning is weak and insufficient. At the next step the stolen force of a final cause comes in so vigorously as to give coloring to the whole argument. This purpose of imitation being accomplished, it cannot fail of the results referred to it. This it is which is relied on to make both branches of the argument acceptable.