Organisms, as has been said, are distinguished from one another, not only in that one is simpler and another more complicated, but also in that those standing at the same stage of organization are unequally differentiated in their functions and in their structure, which is connected primarily with certain external relations which Nägeli calls adaptations. Adaptation appears at each stage of the organization, which stage is, for a given environment, the most advantageous expression of the main type that was itself produced by internal causes. For this condition of adaptation, a sufficient cause is demanded, and this is, as Nägeli tries to show later, the result of the inherited response to the environment. In many cases this cause will continue to act until complete adaptation is gained; in other cases, the external conditions give a direction only, and the organism itself continues the movement to its more perfect condition. The difference between the conception of the organic kingdom as the outcome of mechanical causes on the one hand, or of competition and extermination on the other hand, can be best brought out, Nägeli thinks, by the following comparison of the two respective methods of action. There might have been no competition, and no consequent extermination in the plant kingdom, if from the beginning the surface of the earth had continually grown larger in proportion as living things increased in numbers, and if animals had not appeared to destroy the plants. Under these conditions each germ could then have found room and food, and have unfolded itself without hinderance. If now, as is assumed to be the case on the Darwinian theory, individual variations had been in all directions, the developmental movement could not have gone beyond its own beginnings, and the first-formed plants would have remained swinging now on one side and now on another of the point first reached. The whole plant kingdom would have remained in its entirety at its first stage of evolution, that is, it would never have advanced beyond the stage of a naked drop of plasma with or without a membrane. But, according to the further Darwinian conception, competition, leading to extermination, is capable of bringing such a condition to a higher stage of development, since it is assumed that those individuals which vary in a beneficial direction would have an advantage over those that have not taken such a step, or have made a step backward.