690 Journal of Agriculture . [10 Xov., 19 10. 



not yet dead and the attempt to check the spread of plagues and pestilences 

 is largely regarded as an irreligious interference with the decrees of 

 Providence. 



A partial survival of the same view is evidenced by the undue import- 

 ance attached in comparatively recent years to natural agencies as being 

 responsible for the spread of weeds, pests and diseases, taking the term 

 ■' natural agencies " to mean all factors not connected directly or indirectly 

 with the presence and activity of man, that is to say, those factors that 

 would act in his absence as well as in his presence. Among such agencies, 

 wind, water and animals are the most active agents in dispersal, and to 

 any one familiar with the minuteness of the germs of most animal diseases 

 and of many plant diseases, as well as the special adaptations of the seeds 

 of many weeds to suit them for aerial navigation, it w'ould, at first sight, 

 not seem necessarv to look further for an explanation of the spread of 

 pests and plagues. When, at the same time, it was assumed that such 

 diseases as consumption and cancer in animals and rust and smut in 

 plants were hereditarv in the sense of being transmitted directly to the 

 offspring, while still attached to the parent organi.sm, the outlook seemed 

 indeed to be a liopeless one. 



As a matter of fact, although natural agencies do take part in the 

 spread of weeds and diseases, they do so to a much less extent than might 

 be expected. It is for the most part, by the ill-regulated and unscientific 

 activities of man that most weeds and diseases are spread or that the 

 conditions are ])rovided for Iheir excessive development. Under natural 

 conditions, in the absence of man, there is much less migration of living 

 organisms than might be expected and the term " living organism " of 

 course includes all disease organisms and pests. Were this not the case, 

 no continent could have tleveloped an endemic fauna and flora largely 

 peculiar to itself, and instead of the native flora of Australia being quite 

 distinct from that of any other part of the world, it w'ould have merely 

 n-peated that of corresponding regions in the Northern Hemisphere. That 

 migration has taken place in the past in the absence of man is quite 

 ■certain, hut it probably has always been an extremely slow and gradual 

 process requiring thousands of vears for the introduction and naturalization 

 of a few plants and animals in a new continent. When we compare with 

 this the fact that, during the short period of occupancy of Victoria by 

 civilized man. not far short of four hundred foreign weeds have succeeded 

 in permanentlv establishing themselves in the country, it is easy to see 

 how relatively unimportant are natural agencies for dispersal when unaided 

 or unaffected by man. In other words, man himself is largely responsible 

 directlv or indirectly for the luxuriance and spread of the pests and 

 plagues luider which he suffers, and it is in the intelligent and scientific 

 control of his activities that the best remedies for these evils will be found 

 in the future. 



In this connexion it mav be of interest to mention a few specific 

 instances showing that weeds do not invade an untouched native flora 

 •until man provides the conditions for their development, as well as carrying 

 their seeds or causing them to be carried to the new soil. In the National 

 Park at Wilson's Promontory, along the tram track used by the saw- 

 millers manv vears ago at Sealer's Cove, thistles grow in abundance 

 -without being a!)le to spread into the untouched native flora right and left 

 of the track. The track is some miles in length and at its upper end in 

 several cleared spots and where fires have been, thistles have entered and 



