been a very extended one, stretching through many millions of years, and living matter has passed through an extraordinary diversity of forms, from microscopic specks of primeval jelly to the highly organized form of man. Of any planet upon which thinking beings have appeared, doubtless much the same may be said. The beginning must have been at the same low level; the gradations must have been similar in general character; the ultimate may perhaps have been widely different, though there are what seem good reasons for believing that it was closely accordant.
The final result of organic evolution depends largely upon external relations, the environment; largely upon the relations of organic matter to the chemical conditions of this environment. In certain particulars this has remained persistent throughout. The presence of water and air and the active oxidation of organic substance have been essential conditions of plant and animal existence through all the earth's life era. In other particulars the environment has varied immensely. At first physical only, it soon became in large measure vital. Organic beings, at first struggling for existence against adverse inorganic conditions, soon had to add to this a struggle against one another. As life grew more complex and diversified, so did the vital environment. The hurtful or helpful effects of heat and cold, storm and calm, poisonous and nutritious food, and other inorganic agencies, became of minor importance as agents in evolution in comparison with the intense competition for the food supply between living forms. The development of the carnivorous appetite in animals, with the subsequent necessity of methods of escape or defense in food forms, has been the most prominent selective agency in organic evolution, and the one to which we mainly owe the great diversity of advanced forms now existing. The struggle has been not alone between higher assailants and lower fugitives. It has also taken the form of the assault of lower upon higher forms. And it is of great interest to find that man, the highest of all, finds his most dangerous organic foes in the disease-producing microbes, among the lowest forms of life.
Life, then, in its progress upward, has moved in a somewhat narrow lane, whose borders it could not cross without encountering death. And in dealing with earthly evolution, we are in great measure dealing with evolution everywhere; since, whatever the organic conditions and the inorganic environment, the vital struggle for existence must have been much the same in all life-containing spheres. Nature may be held to have tried a great experiment upon the earth, carried on through a vast stretch of time, as if with intent to discover what ultimate result would arise from this long-continued action of inorganic and organic forces upon living forms.
This experiment has not lacked a sufficiency of material. During
