natural conditions which must be met by every living creature if it is to succeed in life.  In detail, the struggle for existence is intra-specific, involving some form of competition or rivalry among the members of a single species; it is inter-specific, as a conflict is waged by every species with other kinds of living things; and finally it involves an adjustment of life to inorganic environmental influences.  While it may seem unjustifiable to speak of heat and cold and sunlight as enemies, the direct effects produced by these forces are to be reckoned with no less certainty than the attacks of living foes.

The three divisions of the struggle for existence are so important not only in purely scientific respects, but also in connection with the analysis of human biology, that we may look a little further into their details, taking them up in the reverse order.  Regarding the environmental influences, the way that unfavorable surroundings decimate the numbers of the plants of any one generation has already been noted, and it is typical of the vital situation everywhere.  English sparrows are killed by prolonged cold and snow as surely as by the hawk.  The pond in which bacteria and protozoa are living may dry up, and these organisms may be killed by the billion.  Even the human species cannot be regarded as exempt from the necessity of carrying on this kind of natural strife, for scores and hundreds die every year from freezing and sunstroke and the thirsts of the desert.  Unknown thousands perish at sea from storm and shipwreck, while the recorded casualties from earthquakes and volcanic eruptions and tidal waves have numbered nearly one hundred and fifty thousand in the past twenty-eight years.  The effects of inorganic influences upon all forms of organic life must not be underestimated in view of such facts as these.

In the second place, the vital struggle includes the battles of every species with other kinds of living things whose interests are in opposition.  The relations of protozoa and bacteria, conger-eels and other fish, English sparrows and hawks, plants and herbivorous animals, are typical examples of the universal conflict in which all organisms are involved in some way.  Again it is only too evident that human beings must participate every day in some form of warfare with other species.  In order that food may be provided for mankind the lives of countless wild organisms must be sacrificed in addition to the great numbers of domesticated animals reared by man only that they may be destroyed.  The wolf and the wildcat and the panther have disappeared from many of our Eastern states where they formerly lived, while no longer do vast herds of bison and wild horses roam the Western prairies.  Because one or another human interest was incompatible with the welfare of these animals they have been driven out by the stronger invaders.