Popper also recognizes that scientists will work with theories that have been "falsified" and will not in fact discard a theory if there is no better alternative. Scientists are also able to circumvent the falsification of a hypothesis or theory by adding or modifying hypotheses on an ad hoc basis. Popper will allow the addition of an ad hoc hypothesis, however only under the condition that it is independently testable (Bird, 1998, p242-243). However, it seems that in general, if scientists have working theories, they do not abandon them, even in the presence of falsifying evidence. They simply continue to work with the theory until a better alternative arrives (Ladyman, 2002, p89). Scientists have not observed Popper's criterion of falsifiability, and his ideas of science, hypothesis, and theory when applied to science are very restrictive. Bird uses the example of Darwin believing that his "view [of evolution] is confirmed by the fossil record", but that the absence of such evidence would not falsify his hypothesis: the failure to find evidence to support his theory would make no negative difference. (Bird, 1998, p178). This is an example of a type of hypothesis that cannot be falsified called an existential statement. These are statements that predict the existence of something, but by not finding them we cannot falsify their existence, since it could be our mistake