induced by climatic change. But still more strange and striking changes have been recorded as due to external conditions. Thus it is said[31] that certain branchiopodous creatures of the crab and lobster class (certain crustacea) have been changed from the form characteristic of one genus (_Artemia salina_) into that of quite another (_Branchipus_), by having been introduced in large numbers by accident into very salt water. The latter form is not only larger than the former, but has an additional abdominal segment and a differently formed tail. Such changes tell strongly in favor of the existence in creatures of positive, innate tendencies to change in definite directions under special conditions. [30] _Tropical Nature_, pp. 254-259. [31] _Nature_, 1576, June 8, p. 133. Schmankevitsch at Odessa. It is also obvious that the very same influences (_e.g._ amounts of light, heat, moisture, &c.) will produce different effects in different species, as also that the nature of some species is more stubborn and less prone to variation than that of others. Such, for example, is the case with the ass, the guinea-fowl, and the goose, as compared with the dog, the horse, the domestic fowl, and the pigeon. Thus both the amount and the kind of variability differ in different races, and such constitutional capacities or incapacities tend to be inherited by their derivative forms, and so every kind of animal must have its own inherent powers of modifiability or resistance, so that no organism or race of organisms _can_ vary in an absolutely indefinite manner; and if so, then unlimited variability must be a thing absolutely impossible. The foregoing considerations tend to show that every variation is a function[32] of “heredity” and “external influence”—_i.e._ is the result of the reaction of the special nature of each organism upon the stimuli of its environment. [32] In the mathematical sense of the word. In addition to the action of heredity and the action of the environment, there is also a peculiar kind of action due to an internal force which has brought about so many interesting cases of what is called “serial and lateral homology” which cannot be due to descent, but which demonstrate the existence of an intra-organic activity, the laws of which have yet to be investigated. Comparative anatomy, pathology, and teratology combine to point out the action of this internal force. “Lateral homology” refers to the production of similar structures on either side of the body, as in the similarity of our right and left hands and feet. “Serial homology” refers to the production of similar structures one behind the other, as in the series of similar segments in the body of a worm or a centipede, and the similar series of limbs in the latter animal.