ECOLOGICAL 219 



the fluid of its virulence, which supports the view that this depends 

 on the presence of micro-organisms. 



Returning for a moment to foot-and-mouth disease in cattle, 

 which has cost Britain alone many miUions of pounds in a single 

 year (1923-1924), we should notice that several investigators have 

 claimed success in removing it from the list of filterable viruses. 

 In other words, some believe that the micro-organism has been 

 discovered. Thus, to take a well-documented claim (1924), Profs. 

 Frosch and Dahmen, of the Veterinary College in Berlin, report 

 having detected and photographed the microbe (or its halo) by 

 means of the ultra-microscope. They describe it as an exceedingly 

 minute bacillus, not very far from the diphtheria bacillus. A culture 

 has been prepared which produced characteristic blisters in guinea- 

 pigs and partial attacks have been produced in cattle by inoculation. 

 Dahmen speaks of having attenuated the virulence of the virus, 

 and everyone wishes him success in finding a serum which will 

 render cattle immune to attack. We have lingered over this par- 

 ticular case because it illustrates the tenacious persistence and 

 the progress of scientific inquiry even when the problem is peculiarly 

 difficult, as is the case with foot-and-mouth disease. 



In the spreading of "mosaic disease", so-called from the pattern 

 shown by the infected leaf, certain sap-sucking and leaf-eating 

 insects are known to effect the transference from plant to plant. 

 A good illustration of wheels within wheels is afforded by the fact 

 that insects themselves fall victim to the disease. Thus the wilt 

 diseases of the caterpillars of the Nun Moth and the Gypsy Moth 

 are due to filterable viruses, and these two cases are interesting 

 because the results are in man's favour, not against him. The Gypsy 

 Moth caterpillar was accidentally introduced into America in 1869, 

 and has done terrible damage in defoliating trees. The experts seem 

 to think that the appearance of wilt disease has done more towards 

 the eradication of the pest than all man's efforts at control, ingenious 

 and energetic as these have been. Yet what a choice of evils! 



THE LIVING EARTH.— We owe to Pasteur the first vivid realisa- 

 tion of the numbers of bacteria in the soil, but long before that 

 there was some recognition of the "underworld" fauna. Thus 

 Gilbert White (about 1777) recognised very clearly that the number 

 of earthworms was very large, and that the work they have done 

 and do is of far-reaching importance. Gilbert White's picture was 

 repainted by Darwin in his well-known masterpiece. Yet we venture 

 to say that one of the gains of twentieth-century ecology has been 

 a realisation of the literal accuracy of the phrase "the Living 

 Earth". There is, for instance, an invisible army of soil-Protozoa 

 that we are only beginning to know; and we are far from having 

 an adequate knowledge of the numerous and diverse insect larvse 



