12 ZOOLOGY 



lives of animals more closely we become impressed with the 

 tremendous competition which goes on between different kinds 

 of animals. If all the young born in any one species were 

 to survive and grow up, that species would in a very few 

 years overrun the entire surface of the globe. In the case of 

 pheasants it has been calculated that if all the eggs which a 

 pheasant laid were to be hatched and all the young chicks 

 were to survive to maturity and lay the normal number of 

 eggs in turn, a period of eighteen years would suffice to pro- 

 duce from the offspring of a single pair a crowd of pheasants 

 which, if placed side by side touching each other, would cover 

 the entire surface of the globe including the sea, and even 

 then a large number would be left over. But there are many 

 species of animals far more prolific than the pheasant; the 

 cod, for instance, lays nine million eggs in one season, and the 

 only thing which prevents all the food material in the sea 

 being made into young cod in the course of a year or two 

 is the enormous destruction which the young cod are subject 

 to at the hands of their innumerable enemies. 



We may make the matter a little clearer to ourselves if 

 we compare the earth to a huge prairie of dry grass and com- 

 pare each species of animal to a fire which has been lighted in 

 it. Each fire tends to spread outwards so as to overrun the 

 whole prairie, but each is limited in the possibilities of its 

 spread by the spread of its neighbours. One may notice on 

 the banks of the Clyde an oyster- catcher busily gaining its 

 living. This is a bird which feeds largely on the shellfish 

 which are exposed at low tide. The question rises in one's 

 mind, why are not all the rocks denuded of their molluscan 

 inhabitants by this bird? The only answer is that the 

 nestlings of the oyster-catcher are in turn destroyed by some 

 enemy, and that oyster-catchers are thus prevented from 

 becoming inconveniently numerous. It sometimes occurs that 

 a species of animal reaches a new country where its enemies 

 are too feeble to seriously thin its numbers. Then an enor- 

 mous multiplication occurs, and the species spreads in such 

 a manner as to threaten to crush out all other forms of life, 

 If it finally succeeded in doing this it would no doubt bring 

 its own existence to an abrupt termination, for it would 

 exhaust its own food supply. The English rabbit transported 



