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other vertebrates had until then not been able to conquer. Such changes, involving the improvement of the all-round achievements of the organism without depriving it of valuable possibilities, may properly be called biological progress. They are simply examples of specialization that is not one sided, but balanced.

We may take one further example, which brings out the difference between the two processes. The most primitive members of the group to which we and all other backboned animals belong-forms like Amphioxus, for instance—have no true eye, have probably only a very slight sense of smell (certainly no nasal organ of our type), and no ear. The lower vertebrates, such as the fishes, have very efficient sight and smell but practically no sense of hearing. Both birds and mammals (in general) have acute hearing and much improved sight. Here there is a real biological advance; the efficiency of all three senses has enormously improved, and improved in a balanced way, in passing from Amphioxus to higher vertebrate. But in this same field we may find unbalanced improvement, one-sided specialization. The improvement in the utilization of the sense of sight, which is so obvious in the whole group of monkeys and apes and culminates in man, has been accompanied by a degeneration in the power of smell; the same has been true in many birds, which also rely almost entirely upon sight. On the other hand, the mole relies almost entirely upon touch and hearing, and its eyes have degenerated. Thus in all these forms an unbalanced improvement in one direction has led to a cutting down of faculty in another.

The main improvements of life during its evolution must obviously be improvements of the balanced type, not mere specializations, since it seems certain that no highly specialized animal or plant has ever succeeded in becoming the