The changes which organisms undergo, whether sudden or gradual, and whatever their approximate exciting cause, take place in pursuance of the evolution of organic nature, and there can be no doubt that this proceeds under the guidance of law. Professor Owen expresses this fact in saying that “generations do not vary accidentally in any and every direction, but in preordained, definite, and correlated courses.”[387] This may be accepted as expressing a general truth, subject to some qualification of the word “preordained.” It is not exactly true, however, for variations are not always regular and orderly. Within certain limits, indeed, they would seem to take place in any direction, but there is always a tendency for them to accumulate in that course along which they meet with the least resistance. This is in accordance with the principle laid down by Mr. Herbert Spencer, that everything tends towards equilibration, the state being one not of absolute but of moving equilibrium, while “throughout evolution of all kinds there is a continual approximation, and more or less complete maintenance of this moving equilibrium.”[388] The ultimate result is that, “when through a change of habit or circumstance an organism is permanently subject to some new influence, or different amount of an old influence, there arises, after more or less disturbance of the old rhythms, a balancing of them around the new average conditions produced by this additional influence.”[389] It is evident that the variations which have been originated before the attainment of the state of temporary stability thus established would have little chance of being perpetuated; and we have probably here the explanation of the fact that the progress of evolution reveals itself so often by sudden movements. In these cases, where the disturbing influence has rendered the equilibrium of the organism affected more or less unstable, a new centre of equilibrium will be formed, and the appearance of a fresh specific form be the result.