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To resume our pressure analogy, the natural increase of all organisms leads to a “biological pressure.” So long as a species remains unchanged, so long must it stay subjected to the full force of this pressure. But if it changes in such a way that it can occupy a new niche in environment, it is expanding into a vacuum or a region of lower pressure. Natural increase soon fills this up to the same level of pressure, and conditions thus become favourable for expansion into new low-pressure areas previously out of reach of the normal range of variation. Variation towards such “low-pressure” regions may be progressive, retrogressive, or neutral: but it is obvious that at each stage of evolution there will always be a low-pressure fringe, representing a considerable fraction of the “low-pressure” area within the range of variability, the occupation of which would be biologically progressive.

Thus from the well-established biological premisses of (1) the tendency to geometrical increase with consequent struggle for existence, (2) some form of inherited variability, we can deduce as necessary consequence, not only the familiar but none the less fundamental fact of Natural Selection, but also the almost neglected fact that a certain fraction of the guiding force of Natural Selection will inevitably be pushing organisms into changes that are progressive.