502 POPULAR SCIENCE MONTHLY. 



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 organ- 

 ized 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 sim- 

 ilar 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 mat- 

 ter to the chemical conditions of this environment. In certain partic- 

 ulars this has remained persistent throughout. The presence of water 

 and air and the active oxidation of organic substance have been essen- 

 tial 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 con- 

 ditions, 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, poi- 

 sonous and nutritious food, and other inorganic agencies, became of 

 minor importance as agents in evolution in comparison with the in- 

 tense competition for the food supply between living forms. The devel- 

 opment 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 exist- 

 ing. 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 ulti- 

 mate result would arise from this long-continued action of inorganic 

 and organic forces upon living forms. 



This experiment has not lacked a sufiiciency of material. During 



