单项选择题

There can be few more depressing stories in the entire history of man’s exploitation of nature than the destruction of the unfortunate great whales. The whales have not only suffered untold cruelty but now face total extermination. Already entire populations have been wiped out, and the only reason why no species has yet been finished off is due to the vastness and inaccessibility of the oceans; a pocket or two somewhere has always managed to escape. How ironic of biological extinction were to complete the job.
The basic rule of extinction is very simple: it occurs when a species mortality is continually greater than its recruitment. There are tough, some very special additional factors in the case of whales.
Man does not actually have to kill the last whales of a species with his own hands, as it were, to cause its disappearance. Biological extinction will quickly follow the end of commercial whaling, should that end be due to a shortage of raw material, i.e. of whales. Whalers have long sought to defend their wretched trade by insisting that whales are automatically protected: as soon as they become rare, and therefore uneconomic to pursue, man will have no choice but to stop the hunting. That is a very nice theory, but it is the theory of an accountant but not of a biologist; only an accountant could apply commercial economist to complex biological systems. The reasons for its absurdity are many and varied. In the case of whaling it can be summed up in the following way. When the stock has been reduced below a critical level, a natural, possible unstoppable downward spiral begins because of three main factors. First, the animals lucky enough to survive the slaughter will be too scattered to locate one another owing to the vastness of the oceans. Secondly, whales being sociable animals probably need the stimulus of sizeable gatherings to induce reproductive behavior (which has social inferences as well as sexually). It is quite likely that two individuals meeting through chance will not be compatible. (That can hardly be expected to be aware of their own rarity or to realize any need for adjusting their natural inclinations. ) This is especially so with polygamous species like the Sperm Whale. Thirdly, an perhaps most important in the long term, even allowing that the whales might still be able to band together in socially acceptable groups (thanks to their undeniably excellent communicative systems), there is a real danger, possibly even a probability, that the whales’ gene pools would by then have sunk so low as to be biologically unviable. That’s to say, the characteristics possessed by the original population would be whittled down to those characters possessed by only the few remaining individuals. The result of such a biological calamity is inbreeding, less ability to adapt to new conditions, and less individual variety. Three words can sum it up: protracted biological extinction. The future "hopes" of these animals are another issue being heatedly debated by more and more nowadays.
We may infer from the passage that the present situation of whales is that ______.