the self-fertilised plants of the eighth generation of Mimulus,—­or the offspring from flowers on the same plant,—­and in this case a cross between the two plants of Digitalis would have done no good.  But seeds are often widely dispersed by natural means, and one of the above two plants or one of their ancestors may have come from a distance, from a more shady or sunny, dry or moist place, or from a different kind of soil containing other organic or inorganic matter.  We know from the admirable researches of Messrs. Lawes and Gilbert that different plants require and consume very different amounts of inorganic matter. (12/10.  ’Journal of the Royal Agricultural Society of England’ volume 24 part 1.) But the amount in the soil would probably not make so great a difference to the several individuals of any particular species as might at first be expected; for the surrounding species with different requirements would tend, from existing in greater or lesser numbers, to keep each species in a sort of equilibrium, with respect to what it could obtain from the soil.  So it would be even with respect to moisture during dry seasons; and how powerful is the influence of a little more or less moisture in the soil on the presence and distribution of plants, is often well shown in old pasture fields which still retain traces of former ridges and furrows.  Nevertheless, as the proportional numbers of the surrounding plants in two neighbouring places is rarely exactly the same, the individuals of the same species will be subjected to somewhat different conditions with respect to what they can absorb from the soil.  It is surprising how the free growth of one set of plants affects others growing mingled with them; I allowed the plants on rather more than a square yard of turf which had been closely mown for several years, to grow up; and nine species out of twenty were thus exterminated; but whether this was altogether due to the kinds which grew up robbing the others of nutriment, I do not know.

Seeds often lie dormant for several years in the ground, and germinate when brought near the surface by any means, as by burrowing animals.  They would probably be affected by the mere circumstance of having long lain dormant; for gardeners believe that the production of double flowers and of fruit is thus influenced.  Seeds, moreover, which were matured during different seasons, will have been subjected during the whole course of their development to different degrees of heat and moisture.

It was shown in the last chapter that pollen is often carried by insects to a considerable distance from plant to plant.  Therefore one of the parents or ancestors of our two plants of Digitalis may have been crossed by a distant plant growing under somewhat different conditions.  Plants thus crossed often produce an unusually large number of seeds; a striking instance of this fact is afforded by the Bignonia, previously mentioned, which was fertilised by Fritz