106 The Outline of Science 



the juvenile stages of the liver-fluke of the sheep. The mosquito 

 is the vehicle of malaria from man to man, and the tse-tse fly 

 spreads sleeping sickness. The freshwater mussel cannot con- 

 tinue its race without the unconscious co-operation of the min- 

 now, and the freshwater fish called the bitterling cannot continue 

 its race without the unconscious co-operation of the mussel. 

 There are numerous mutually beneficial partnerships between 

 different kinds of creatures, and other inter-relations where the 

 benefit is one-sided, as in the case of insects that make galls on 

 plants. There are also among kindred animals many forms of 

 colonies, communities, and societies. Nutritive chains bind long 

 series of animals together, the cod feeding on the whelk, the whelk 

 on the worm, the worm on the organic dust of the sea. There is 

 a system of successive incarnations and matter is continually 

 passing from one embodiment to another. These instances must 

 suffice to illustrate the central biological idea of the web of life, 

 the interlinked System of Animate Nature. Linnaeus spoke of 

 the Systema Naturae, meaning the orderly hierarchy of classes, 

 orders, families, genera, and species; but we owe to Darwin in 

 particular some knowledge of a more dynamic Systema Natura?, 

 the network of vital inter-relations. This has become more and 

 more complex as evolution has continued, and man's web is most 

 complex of all. It means making Animate Nature more of a 

 unity; it means an external method of registering steps of pro- 

 gress ; it means an evolving set of sieves by which new variations 

 are sifted, and living creatures are kept from slipping down the 

 steep ladder of evolution. 



Parasitism 



It sometimes happens that the inter-relation established be- 

 tween one living creature and another works in a retrograde di- 

 rection. This is the case with many thoroughgoing internal 

 parasites which have sunk into an easygoing kind of life, utterly 

 dependent on their host for food, requiring no exertions, running 



