400 THE BIOLOGY OF BIRDS 



make good for such disastrous interferences as introducing 

 rabbits into Australia or sparrows into the United States. 

 What we plead for is careful scientific consideration of every 

 step of interference ; what we protest against is ever)^hing 

 in the way of drastic, artificial, wholesale elimination, 

 as implied, for instance, in " big bags," for that is justified 

 neither by science nor sport ! 



" There is an Australian story which reads as if written 

 for man's instruction. On certain Murray River swamps 

 several species of cormorants used to swarm in thousands, 

 but ruthless massacres, based on the supposition that the 

 cormorants were spoiUng the fishing, reduced them to 

 hundreds. But the fishing did not improve ; it grew worse. 

 It was then discovered that the cormorants feed largely 

 on crabs, eels, and some other creatures which devour the 

 spawn and fry of the desirable fishes. Thus the ignorant 

 massacre of the cormorants made for the impoverishment, 

 not for the improvement, of the fishing. The obvious 

 moral is that man should get at the facts of the web of 

 life before, not after, he has recourse to drastic measures 

 of interference " (Thomson, 1919, p. 88). 



As regards the toll that birds levy on the lower Verte- 

 brates (notably, Amphibians and Fishes), and on many 

 Invertebrates (such as molluscs, spiders, insects, crustaceans 

 and worms), we may safely say that the lower creatures can 

 afford it. So far as one can understand the architecture 

 of Nature, the abundance of minor creatures seems to form 

 the foundation for higher expressions of life. The mosquito 

 makes the swallow possible, and the midge the swift. This 

 is not saying that mosquitos and midges were created in 

 order that there might be swallows and swifts, yet the 

 abundance of life among the lower forms of life has, as a 

 matter of fact, made higher life possible. 



Some of the tangles in the web of life are curious. 

 Squirrels are often shot down because they destroy so many 

 young trees, but an over- elimination of squirrels leads to 

 an over-multiplication of wood-pigeons, on whose young 

 squabs the ordinarily vegetarian squirrel levies useful 



