According to the passage, ‘the bitter truth’ refers to:

Science and the techniques to which it has given rise have changed human life during the last hundred and fifty years more than it had been changed since men took to agriculture, and the changes that are being wrought by science continue at an increasing speed. There is no sign of any new stability to be attained on some scientific plateau. On the contrary, there is every reason to think that the revolutionary possibilities of science extend immeasurably beyond what has so far been realized. Can the human race adjust itself quickly enough to these vertiginous transformations, or will it, as innumerable former species have done, perish from lack of adaptability? The dinosaurs were in their day the lords of creation, and if there had been philosophers among them, not one would have foreseen that the whole race might perish. But they became extinct because they could not adapt themselves to a world without swamps. In the case of man and science there is a wholly new factor, namely that man himself is creating the changes of environment to which he will have to adjust himself with unprecedented rapidity. Although these changes come about through human agencies, they have, or at any rate have had so far, something of the inexorable inevitability of natural forces. Whether men will be able to survive the changes of environment that their own skill has brought about is an open question. If the answer is to be in the affirmative, men will have to apply scientific ways of thinking to themselves and their institutions.