There are many other instances which may be quoted of the co-operation of fungi with other organisms, and it is only of late years that we have appreciated the fact of _symbiosis_, or the living together of two organisms for the mutual benefit of each. This fact was first pointed out in so-called lichens, which are now shown to be complex bodies consisting of a fungus and an alga, living in symbiotic community for the mutual benefit of each. It was next shown that the papilionaceous leguminosæ are unable to flourish without certain bacterial nodules which grow upon their roots, and by the instrumentality of which they can appropriate the nitrogen of the air, and thus the fact, familiar for centuries, that the leguminosæ leave the ground in a state of great fertility, while they are singularly independent of nitrogenous manures, has been explained. But if the plants themselves are independent of dung, it is not so, apparently, with the symbiotic nodules, which seem to flourish far more vigorously in rich garden ground than they do in comparatively poor farm land. Thus Sir John Lawes has grown clover in a rich old garden for forty-two years, and has had luxuriant crops every year. According to my own observation on the scarlet runner bean these nodules are more plentiful upon the roots which grow superficially than upon those which run deeply. Symbiosis is observable in many plants other than the leguminosæ, and it is certain that many of our big forest trees depend for their nourishment upon fungi which grow upon their roots. All animals appear to be symbiotic, for we all carry about millions of microbes, which must fairly be regarded as junior partners in our economy, and which we cannot do without. The microbe which has been chiefly studied—the _Bacterium Coli commune_—appears to be essential for certain digestive processes which go on in the intestines while we live; and when we die, this microbe is active in starting the dead body upon that cycle of events which is one form of the 'Circulation of Organic Matter.' Now it is certain that the dung of all animals swarms with bacteria and allied organisms when it leaves the intestines, and it seems highly probable that excrement carries with it the biological machinery which is necessary for its dissolution and ultimate humification. My friend, Mr. George Murray, F.R.S., the keeper of the Botanical Department of the British Museum, whose learning in fungology is well known, has kindly furnished me with an elaborate list of 139 genera of fungi which flourish on excrement. Of these 139 genera, Mr. Murray has tabulated no less than 628 species which are known to flourish on excrement. Of the 628 species, 226 have been found on the dung of more than one genus of animals, but no less than 402 species of fungi are peculiar to the excrement of only one genus of animals.