chances of their giving rise to a new colony when that migration is successfully accomplished. Every head of the Reed-mace liberates about a quarter of a million seeds of marvellous lightness; yet the Reed-mace does not increase in the country, nor is it a particularly abundant plant even in its chosen habitats. The Foxgloves (_Digitalis purpurea_) in a wood shed, each plant, say a hundred thousand seeds; yet on an average only one of these attains maturity, otherwise the species would become more abundant in the area. This enormous destruction of seed is largely due to competition. The reception which a plant receives in its new home is the thing that matters, and that may usually be summed up in the phrase “House full.” Nevertheless, the present flora of Great Britain is in the long run the result of migration from surrounding areas; so that ease of dispersal has undoubtedly played its part in the building up of our vegetation. Conditions under which rapid dispersal has obviously an advantage occur when by some exceptional circumstances the natural vegetation is destroyed within an area, as by a flood or landslide. Such conditions are produced artificially each season over much of our own country by the operations of agriculture. Their results will be considered in a subsequent chapter. CHAPTER IV SOME INTER-RELATIONS OF PLANTS AND ANIMALS The most important and fundamental difference between the animal and plant worlds is this: plants possess the power of manufacturing their food out of the inorganic materials of which it is composed, while animals cannot do this. Give an ordinary plant access to water with a pinch of mineral salts in it, to the air, and to sunlight, and by the agency of chlorophyll--the green colouring-matter of the leaves--the miracle will be accomplished, and dead materials transformed into living substance. Animals, on the other hand, are dependent for their food-supply on organic material--that is, on either plant or animal substances; and since they cannot live by taking in each other’s washing--in other words, by eating each other--it follows that the animal world is dependent on the plant world for its continued existence. A porpoise may live on herrings, herrings on small fry, fry in turn on minuter organisms, and so on down the scale; but their ultimate source of food is the tiny Algæ which swarm in the water--the _Plankton_ in Hensen’s original sense--which, alone in this chain, can build up their bodies out of the sea and air. That these minute plants can sustain the enormous drain upon them due to their use as a food-supply by myriads of larger organisms is due to their vast numbers and rapid increase. Sea-water favourable for plankton life may contain several millions of individuals in every litre (about 1-3/4 pints); while as a fair estimate for the seas which surround our own islands “at least one” organism for every drop has been suggested.[6]