Scattered about Hampstead Heath are a number of little pools, not big enough to be dignified by the name of ponds. They are generally surrounded by furze bushes, and would escape attention if not actually looked for. Those which are mere puddles, and have only a brief existence in rainy weather, seldom reward the labour of investigation; but others are permanent, except after prolonged drought, and afford convenient situations for the growth of confervæ, star-weed, and other plants. These will nearly always repay the microscopic collector during the winter, when he must break the ice to get at their contents; in spring, when long chains of frog-spawn afford ocular evidence of the prolific properties of the Batrachian reptiles; and in summer, when they afford both shade and sunshine to their numerous inhabitants. Small beetles, water-spiders, larvæ of gnats, and other insects, rotifers, including the tubicolar sorts, and several varieties of infusoria may be expected and generally found. There is, however, a curious fact about ponds, big and little, which Pritchard remarks upon in his 'Infusoria,' and which corresponds with our own experience, that those which have proved to be well stocked with any particular creature during one year, will very likely contain none of it in the next. There are of course exceptions to this rule, but we have often been astonished and disappointed at finding the complete change, both in populousness and population, that a revolution of twelve months will make; and it would be extremely interesting to notice the changes that took place during a term of years.