The preceding examples and discussion give some idea of what is meant by adaptation in living things. In what respects, it may be asked, do these adaptations differ from inorganic phenomena? The first group of inorganic bodies that challenges comparison are machines. These are so constructed that they may be said to accomplish a definite purpose, and the question arises whether this purpose can be profitably compared with the purposefulness of the structure and response of organisms. That the two cannot be profitably compared is seen at once, when we recall the fact that the activity of the machine is of no use to it, in the sense of preserving its integrity. The object of the machine is, in fact, to perform some useful purpose for the organism that built it, namely, for man. Furthermore, the activity of the machine only serves to wear it out, and, therefore, its actions do not assist in preserving its integrity as do some, at least, of the activities of an animal. It is true, of course, that in a mechanical sense every action of the organism leads also to a breaking down of its structure in the same way that a machine is also worn out by use; but the organism possesses another property that is absent in the machine, namely, the power of repairing the loss that it sustains. One of the most characteristic features of the organism is its power of self-adjustment, or of regulation, by which it adapts itself to changes in the environment in such a way that its integrity is maintained. Most machines have no such regulative power, although, in a sense, the fly-wheel of an engine regulates the speed, and a water-bath, with a thermostat, regulates itself to a fixed temperature; but even this comparison lacks one of the essential features of the regulation seen in organisms, namely, in that the regulation does not protect the machine from injury. It may be claimed, however, that the safety valve of an engine does fulfil this purpose, since it may prevent the engine from exploding. Here, in fact, we do find better grounds for comparison, but, when we take into account the relation of the regulations in the organism to all the other properties of the organism, we see that this comparison is not very significant. The most essential difference between a machine and an organism is the power of reproduction possessed by the latter, which is absent in all machines. Here, however, we meet with a somewhat paradoxical relation, since the reproductive power of organisms cannot be looked upon as an adaptation for the continuation of the individual, but rather for the preservation of a series of individuals. Hence, in this respect also, we cannot profitably compare the individual with a machine, but if we make any comparison we should compare all the individuals that have come from a single one with a machine. In this sense the power of reproduction is a sort of racial regulation. A comparison of this sort is obviously empty of real significance.