Our profound interest in the problem of survival is natural and practical and inevitable. But in spite of Darwin's great contribution toward a scientific analysis of the mechanism of organic evolution, and in spite of the marvelous recent progress of medicine along its many branches, the fact remains that so far as this interest in the problem of survival is dominant it must continue to hinder adequate analysis of the problem of adaptation. Indeed, it is in large measure due to such domination in the past that biology now lags so far behind the less personal sciences of physics and chemistry. For survival means the survival of an individual. And there is no doubt that the individual organism is the most conspicuous datum in the living world. The few who, neglectful of individuals and survivals, find their chief interest in living substance, its properties and processes, are promptly challenged by the many to find living substance save in the body of an organism. Thus, in a peculiarly significant sense, organisms are vital units. And since the individual organism shows a remarkable capacity to retain its identity under a wide range of conditions, adaptability or adjustability comes to be reckoned as the prime characteristic of life by all to whom the integrity of the individual organism is the fact of chief importance. With the use of the words adaptability and adjustability, our discussion assumes a somewhat different aspect. Instead of contemplating further the mechanical selection of individuals on the basis of characters that, like the structure of "the woodpecker, with its feet, tail, beak and tongue, so admirably adapted to catch insects under the bark of trees," can not be attributed to the influence of the external conditions that render them useful, we are invited to consider immediate and plastic adjustments of the organism to the very conditions that call forth the response. For the fortuitous adjustments that tend to preserve those individuals or races that chance to possess them, are substituted, accordingly, the direct primary adjustments that tend to preserve the identity of the reacting organism. We turn thus from the RESULTS of the selection of favorable variations to the biological PROCESSES by which organisms become accommodated to their conditions of life.