f' 



58 BASIS OF THE PEOBLEM 



liable to be consumed by enemies. Even where copulation is 

 internal, the developing embryo is, except among mammals, 

 seldom long retained within the body of the mother, and once 

 exposed to the outside world it almost invariably becomes an 

 object of prey to many enemies. In fact, wherever we look, this 

 cause of elimination plays a very great part, arising immediately 

 from that aspect of the interdependence of species which is derived 

 from the mode among animals of making up for the wastage 

 of protoplasm. 

 ^ Elimination again is largelj^ due to failure to find those conditions 

 under which alone hfe can continue. These conditions may be 

 connected with the nature of the organic or of the inorganic 

 surroundings. When the young of the flat worm, known as the 

 Liver Fluke, which infects sheep and causes a serious disease, 

 passes from the sheep to the exterior, it can only maintain itself 

 for a certain time in the free-living form. Unless within this 

 time it meets with a certain species of snail into which it penetrates, 

 it will die. Besides such failures to find suitable organic surround- 

 ings, there may be failures to find suitable inorganic surroundings. 

 The larvae of such species as the mussel, which require suitable 

 surroundings to which to attach themselves in order that the 

 adult form may develop, will perish if such surroundings are not 

 available. There is a further class of factors connected with the 

 inorganic surroundings which bring about elimination. They may 

 be summed up under the heading of external circumstances. 

 Variations in temperature, moisture and so on, when they pass 

 beyond a certain limit, which is more or less clearly marked for 

 each species, are followed by death. Under this heading comes 

 also death from accident, as when animals perish from the violence 

 of storms and in any similar fashion. It is of interest to note 

 that starvation is seldom the primary cause of death. It may be 

 a secondary result of abnormal external circumstances ; extreme 

 cold, though not affecting directly the members of one species, 

 may be fatal to members of another species upon which the 

 former feed. But when circumstances are normal, so far as 

 observation goes, starvation is rare. 



15. Among plants the same three groups of factors can be 

 traced, though their relative importance is not the same as among 

 animals. Elimination through consumption as food, for instance, 

 by other species is not so important. To a large extent plants 



