AN interesting effect of the habit is furnished by a plant native to the Mediterranean, which has established itself in northern waters. It is one of the relatively few seaweeds which have acquired common names—the oyster-thief. The plants grow into an irregular sort of bladder on the shells of the young oysters. The activities of the plant produce a gas bubble inside the bladder which will, if growth proceed long enough, suffice to lift young oysters from their moorings. An offshore current will carry them into deep water where they are lost to the oyster-grower. Various methods of combating the nuisance have been tried, but nothing but mechanical bursting of the bladders by sweeping the oyster-beds with a brushwood contrivance has proved of much use so far. What we need to know are the conditions which are favourable to the oyster and unfavourable to the plant. Even if we knew
them it may be doubted if we could find means to have them in combination. It may reasonably be argued that if general conditions, including the population of species previously in occupation of the territory and their relations one with another, were not highly favourable, no intruding species would have a chance of establishing itself.
If man, as an economist, wishes to eliminate a species unfavourable to his plans, he must turn to and record all the species in the vicinity and the limits of their toleration in respect of the master-factors of the environment. He will then be able to see if the species he does not want can be crowded or otherwise driven out. There is nothing very remarkable or mysterious about this : it is the problem which is solved with varying degrees of success by every cultivator, whether of oysters, wheat or cows. Of course, all that we can do, usually, is to destroy the hostile species; as by hoeing weeds or shooting wolves.