Thompson’s transformation grid analysis for the transition from the Eocene Hyracotherium skull form (A) to the modern Equus skull form (B). Note the representation of hypothetical intermediate stages of the transformation via linear interpolation (C). From Thompson 1917.
The linear interpolation is the straight line between the two points which are given by the coordinates (x0, y0) and (x1, y1). In the interval of (x0, x1) the value of x which gives the straight line and it is given from the equation for the value of y along. y−y0x−x0 = y1−y0x1−x0