Only artificial pasture-grounds are seen to consist of a single species of grass or clover. The natural condition in meadows is the occurrence of clumps of grasses and some clovers, mixed up with perhaps twenty or more species of other genera and families. The numerical proportion of these constituents is of great interest, and has been studied at Rothamstead in England and on a number of other farms. It is [103] always changing. No two successive years show exactly the same proportions. At one time one species prevails, at another time one or two or more other species. The weather during the spring and summer benefits some and hurts others, the winter may be too cold for some, but again harmless for others, the rainfall may partly drown some species, while others remain uninjured. Some weeds may be seen flowering profusely during some years, while in other summers they are scarcely to be found in the same meadow. The whole population is in a fluctuating state, some thriving and others deteriorating. It is a continuous response to the ever changing conditions of the weather. Rarely a species is wholly annihilated, though it may apparently be so for years; but either from seeds or from rootstocks, or even from neighboring lands, it may sooner or later regain its foothold in the general struggle for life. This phenomenon is a very curious and interesting one. The struggle for life, which plays so considerable a part in the modern theories of evolution, may be seen directly at work. It does not alter the species themselves, as is commonly supposed, but it is always changing their numerical proportion. Any lasting change in the external conditions will of course alter the average oscillation and the influence [104] of such alterations will manifest itself in most cases simply in new numerical proportions. Only extremes have extreme effects, and the chance for the weaker sorts to be completely overthrown is therefore very small. Any one, who has the opportunity of observing a waste field during a series of years, should make notes concerning the numerical proportions of its inhabitants. Exact figures are not at all required; approximate estimates will ordinarily prove to be sufficient, if only the standard remains the same during the succeeding years.