THE INTERDEPENDENCE OF LIV1\(, Tl||\f;s 



49 



such as tho division of the colony into castes thai include nuih-s 

 (drones), fertile females ((lueens), and infertile females (workers). 

 Castes are even more mmierous among ants, there being winged and 

 wingless females, intermediates between females and workers, soldiers, 

 several groups of workers, and winged and wingless males. Not all of 

 these forms, however, are found in any one species. By means of such 

 division of labor, life in the colony goes on at a very efficient level. 



Animal Cannibalism 



Most of us have had the experience of having some pet destroy 

 her young when they were in danger, or of having laboratory-bred 

 rats or mice eat their newborn young. This is probably a perverted 

 instinct, but nevertheless animal cannibalism is .seen rather fre- 

 quently. The destruction of a wounded member of a pack of wolves 

 when hunting is usual. The female spider usually kills the male 

 after fertilization of the eggs, this habit being common to some 

 other forms. Similarly the eggs may be destroyed by the male, 

 as in the case of the mole cricket and centipede, w^hich eat the eggs 

 shortly after they are laid, the mothers resorting to numerous pro- 

 tective devices in order to thwart the cannibalistic fathers. Many 

 fish eat the eggs of their own species. Even the domestic hen at 

 times will eat her own eggs. 



Relations of Competition 



Evidences of competition in the plant world are numerous. Be- 

 cause of their sessile habit, older plants may overshadow and crowd 

 out the young ones, or one group of plants may prevent the growth 

 of other plants in the vicinity. Weeds and plants in general pro- 

 duce enormous quantities of seed, which are kept from germinat- 

 ing by the rapid growth of the older plants. Many grasses and some 

 shrubs grow rapidly by means of underground shoots, in this way 

 securing territory which might be used by other plants. Thus plants 

 with favorable adaptations may completely pre-empt new territory for 

 themselves at the expense of others le.ss able to use the environment. 



In animals, competition between individuals of a species is almost 

 universal. Males fight each other for the possession of females, or 

 sometimes just for the sake of fighting. There is a contituial struggle 

 for food, for water, and for a place to live. I>arger animals, as we 

 have seen, prey on smaller ones and in general those best fitted to 

 compete in the battle of \Uo, survive. 



