The problem is now being investigated as it affects different
countries ; but there has not been time yet to collect much data from far overseas. My previous contention that the periodicity is much more emphatic in Australia than elsewhere is remarkably illustrated in Professor Baldwin Spencer's book just published. He saw marching armies of rats migrating in hordes across the and plains of Central Australia, exactly as the lemmings migrate in mad almost suicidal masses across Scandinavia. The prevalent theory is that when. the numbers reach a certain height, disease geti a hold and deci- mates or more than decimates the population, as is un- doubtedly the case with other classes of animal, wood pigeons for example. But in regard to mice and rats in Australia, I shOidd say that the disappearance after the surplus is, first,of all, due to this " schwarmerei " and wild migratory impulse, probably due in its origin to the threat of starvatioh. ROdents may migrate as mad,t`y as bees or gossamer spiders.
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