THE NUMBERS OF ANIMALS iii 



animals mostly perish. It is not a rare or exceptional thing for 

 a species to break out of control of its normal checks ; and we 

 shall have to return to this subject again later on, since we shall 

 see that " plagues " of animals are an inevitable consequence 

 of the way in which animal communities are arranged and of 

 the great instability of the environment. 



1 6. Many of the most striking cases of sudden increase in 

 animals occur when a species is introduced into a country 

 strange to it, in which it does not at first fit harmoniously, 

 often with disastrous results to itself or to mankind. 



The most familiar example of such an introduction is that 

 of the rabbit in Australia. It was also introduced into New 

 Zealand, where it multiplied so excessively as to eat down and 

 destroy the grass over wide areas, so that many thousands of 

 sheep died from starvation. ^"'^ In the same way the Gipsy 

 Moth (Lymantria dispar) was introduced into America from 

 Europe, where it became for some years one of the more 

 serious pests in forests, owing to its great increase and 

 spread. It has been shown recently that the increase was due 

 to the absence of its normal parasites, which keep down the 

 numbers in Europe. The introduction of these parasites into 

 America appears to have acted as an effective check, reducing 

 the numbers to reasonable proportions. ^^ 



17. When an animal spreads rapidly in this way upon 

 being introduced into a new country, there is usually a definite 

 sequence of events, which is rather characteristic. At first 

 the animal is unnoticed for several years, or else is highly 

 prized as forming a link with the home country. Thus the 

 starling was introduced into New Zealand by acclimatisation 

 societies bent upon brightening the country with British 

 birds.^^^ The next stage is that the animal may suddenly appear 

 in the dimensions of a plague, often accompanied by a migra- 

 tion, as when huge armies of rats marched over New Zealand 

 in the early days.^^^ The starling was instrumental in spread- 

 ing the seeds of the common English blackberry in New 

 Zealand and has been undoubtedly one of the biggest factors 

 in the production of the blackberry plague there. This has 

 resulted in the formation of thickets of blackberry covering 



