But the causes and conditions of variation have yet to be thoroughly explored; and the importance of natural selection will not be impaired, even if further inquiries should prove that variability is definite, and is determined in certain directions rather than in others, by conditions inherent in that which varies.
When we consider, however, the great variability in those surroundings and the consequent changes a plant must encounter, it appears obvious that interaction ~nd adjustment between the plant and its environment must be constant and well balanced.
It was difficult to be sure as to the variations in the actual number of fish caught, but it was easy to show that there was a real variability in the yield of cod-liver oil (an important product of the fishery).
In both cases no extraneous cause can be assigned; the period seems to be inherent in the star itself and not to be determined by the revolution of a satellite (no variability of the line-of-sight motion of Mira has been found, so that it is probably not accompanied by any large companion).
A transformation which is sometimes rapid, sometimes slow, but always continuous, is wrought by the reciprocal action of the innate variability of plants and of the variability of the external factors.