no ANIMAL ECOLOGY 



collembola or sp ringtails, which lay so thickly on the lines as 

 to cause the wheels of the engines to slip round ineffectually 

 on the rails. The fact that individual springtails are usually 

 about one-twentieth of an inch long will give some idea of the 

 numbers involved. Again, a huge multiplication of water- 

 fleas (Cladocera) took place in the Antwerp reservoirs in 1896 ; 

 the numbers were so serious that six men had to work night 

 and day removing the water-fleas by straining the water 

 through wire gauzes. It was estimated that ten tons of water- 

 fleas were taken out — that is, two and a half times the weight 

 of a large hippopotamus.^^ In the sea there are sometimes 

 " plagues " of protozoa. Peridinians (e.g. Gonyaulax) some- 

 times turn the sea to the colour of blood with their vast 

 numbers, oiT the coast of India, of California, and of 

 Australia.S2 They may be so numerous as to remove most 

 of the free oxygen from the water, so that the fish die from 

 suffocation. Gran once found that the water in Christiania 

 Fjord was milky with a species of coccosphere {Pontosphcera 

 Huxleyi)y which is a microscopic plant; and estimated that 

 there were five to six million per litre.^^ 



14. Finally, there are the diseases caused by various para- 

 sitic animals ; these are nothing more than a breaking away of 

 parasites from the control of the host and increasing at an 

 enormous speed. For example, malaria in the blood, and 

 sleeping-sickness, and all such diseases are the result of over- 

 increase of parasites, just as mouse-plagues are the result of 

 over-increase in mice. The most striking epidemic diseases 

 are of course caused by bacteria or by invisible " viruses,'* 

 but they illustrate the same idea. 



15. We started to describe these examples of enormous 

 multiplication in wild animals in order to emphasise the 

 tremendous powers of increase possessed by them and by all 

 animals. Any species, if given the opportunity, is capable of 

 increasing in the same alarming way as the mice, the locusts, or 

 the Gonyaulax ; and as a matter of fact most species probably 

 do so occasionally, producing plagues which are rather sudden 

 in onset, and which are terminated by disease or some other 

 factors, or else are relieved by migration during which the 



