A more difficult problem than that relating to the sort of changes the external influences bring about in the organism, is the question as to how they effect the organism, or how they act on it mechanically. This, as is well known, was answered by Darwin, who regards all organization as a problem of adaptation: only those chance variations surviving which are capable of existence, the others being destroyed. On this theory external influences have only a negative or a passive action, namely, in setting aside the unadapted individuals. Nägeli, on the other hand, looks upon some kinds of external conditions as directly giving rise to the adaptive characters of the organism. This is accomplished, he supposes, in the following ways: two kinds of influence are recognized; _the direct action_, which, as in inorganic nature, comes to an end when the external influences come to an end, as when cold diminishes the chemical actions in the plant; and _the indirect action_, generally known as a stimulus, which starts a series of molecular motions, invisible to us, but which we recognize only in their effects. Very often the stimulus starts only a reflex action, usually at the place of application. A stimulus acting for but a short time produces no lasting effect on the idioplasm. A person stung by a wasp suffers no permanent effect from the injury. But if a stimulus acts for a long time, and through a large number of generations, then it may, even if of small strength, so change the _idioplasm_, that a tendency or disposition capable of being seen may be the result. This appears to be the case in regard to the action of light, which causes certain parts of the plant to turn toward it and others away from it; also for the action of gravity, which determines the downward direction of the roots. It may be claimed, perhaps, that these are the results of direct influence and not of an internal response, but this is not the case; for some plants act in exactly the opposite way, and send a stem downward, as in the case of the cleistogamous flowers of _Cardamine chenopodifolia_; and other plants turn away from the light. This means that the idioplasm behaves differently in different plants in response to the same stimulus. Concerning the more visible effects of adaptation, Nägeli states that in regard to some of them there can be no question as to how they must have arisen. Protection against cold, by the formation of a thick coat of hair, is the direct result of the action of the cold on the skin of the animal. The different weapons of offence and of defence, horns, spurs, tusks, etc., have arisen, he maintains, through stimulus to those parts of the body where these structures arise.