I shall introduce it to your attention by asking a very simple question. We may admit that digestion, for example, is a purely chemical operation, and one that may be exactly imitated outside the living body in a glass flask. My question is, how does it come to pass that an animal has a stomach?--and, pursuing the inquiry, how does it happen that the human stomach is practically incapable of digesting cellulose, while the stomachs of some lower animals, such as the goat, readily digest this substance? The earlier naturalists, such as Linnaeus, Cuvier or Agassiz, were ready with a reply which seemed so simple, adequate and final that the plodding modern naturalist cannot repress a feeling of envy. In their view plants and animals are made as they were originally created, each according to its kind. The biologist of to-day views the matter differently; and I shall give his answer in the form in which I now and then make it to a student who may chance to ask why an insect has six legs and a spider eight, or why a yellowbird is yellow and a bluebird blue. The answer is: "For the same reason that the elephant has a trunk." I trust that a certain rugged pedagogical virtue in this reply may atone for its lack of elegance. The elephant has a trunk, as the insect has six legs, for the reason that such is the specific nature of the animal; and we may assert with a degree of probability that amounts to practical certainty that this specific nature is the outcome of a definite evolutionary process, the nature and causes of which it is our tremendous task to determine to such extent as we may be able. But this does not yet touch the most essential side of the problem. What is most significant is that the clumsy, short-necked elephant has been endowed--"by nature," as we say--with precisely such an organ, the trunk, as he needs to compensate for his lack of flexibility and agility in other respects. If we are asked _why_ the elephant has a trunk, we must answer because the animal needs it. But does such a reply in itself explain the fact? Evidently not. The question which science must seek to answer, is _how_ came the elephant to have a trunk; and we do not properly answer it by saying that it has developed in the course of evolution. It has been well said that even the most complete knowledge of the genealogy of plants and animals would give us no more than an ancestral portrait-gallery. We must determine the causes and conditions that have cooperated to produce this particular result if our answer is to constitute a true scientific explanation. And evidently he who adopts the machine-theory as a general interpretation of vital phenomena must make clear to us how the machine was built before we can admit the validity of his theory, even in a single case. Our apparently simple question as to why the animal has a stomach has thus revealed to us the full magnitude of the task with which the mechanist is confronted; and it