It has long been known that no definite line of demarcation can be drawn between the animal and the vegetable worlds. There are lowly organisms which cannot be decisively referred either to the one or to the other. It has been more recently shown that the apparently more strongly marked line between the living and the non-living also grows wavering and indistinct in places. Metals are known to respond to stimuli and to show ‘fatigue’ in a manner commonly attributed only to the nervous system of animals,[16] while some of the phenomena of crystallization strongly resemble those of vitality.[17] Le Dantec has uttered the latest word of physics on this subject, where he insists on the “absence of all essential difference and all absolute discontinuity between living and non-living matter.”[18] Indeed, one may say of nature-study in general, that if, as Plato said, the beginning of knowledge is in definitions and classification, the end of it lies in getting rid of them. There is probably no such thing as a universally applicable definition of any group of natural phenomena. _There is certainly no condition of matter of which we are entitled to say that Life is impossible without it._ Still, natural groups have well-marked central features, even if at their margins they melt into something else. Now the things which in the ordinary sense of the word we call Living are marked by these characters: Their chemical constituents are always compounds of carbon. These compounds are what is called ‘unstable’—they ‘consume’ or disintegrate by combining with oxygen in air or water. In this process organisms obtain the energy necessary for assimilation and growth. The above characters (carbon-compounds, chemical instability, and faculty of assimilation) apply to plants and animals alike. But we find, in general, that plants are able, from inorganic mineral constituents such as carbonic acid, water, nitrates, sulphates, etc.,[19] to build up the organic compounds like proteid, albumin, the carbohydrates, alcohol, fat; while animals use for their nourishment not the inorganic substances but only organic compounds already formed by plants or by other animals. A well-developed vegetable world must therefore, it would seem, have preceded the appearance of animal life on the globe.[20] It was long believed that these organic compounds of carbon could only be formed by the vital action of living vegetation. One of the epochs in the history of modern chemistry has been the demonstration (first by Wöhler, in 1828,) that many of them can be produced in the laboratory from inorganic chemical constituents. But this is only effected by circuitous and difficult ways, and—a circumstance often overlooked—it only resembles what is accomplished in nature if we include under nature the directive agency represented by the chemist himself, as well as the materials with which he deals.