Even Popper acknowledges that scientists will work with theories that have been "falsified" and will not actually 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 ad hoc hypotheses. Popper will allow the addition of an ad hoc hypothesis, but only on the condition that it is independently testable (Bird, 1998, p242-243). However, it seems that in general, if scientists have theories that work, they do not abandon them, even in the presence of falsified 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 about science, hypotheses, and theory when applied to science are very restrictive. Bird uses the example of Darwin considering that "his view [of evolution] must be confirmed by fossil remains", but that the absence of such evidence would not falsify his hypothesis – the inability 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, because we might be wrong to believe that