provide plausible links between several different groups of organisms, such as Archaeopteryx linking birds and non-avian dinosaurs, or the Tiktaalik linking fish and limbed amphibians. Creationists dispute such examples, from asserting that such fossils are hoaxes or that they belong exclusively to one group or the other, to asserting that there should be far more evidence of obvious transitional species. Darwin himself found the paucity of transitional species to be one of the greatest weaknesses of his theory: Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain and this, perhaps, is the most obvious and gravest objection which can be urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record. Darwin appealed to the limited collections then available, the extreme lengths of time involved, and different rates of change with some living species differing very little from fossils of the Silurian period. In later editions he added "that the periods during which species have been undergoing modification, though very long as measured by years, have probably been short in comparison with the periods during which these same species remained without undergoing any change." The number of clear transitional fossils has increased enormously since Darwin's day, and this problem has been largely resolved with the advent of the theory of punctuated equilibrium, which predicts a primarily stable fossil record broken up by occasional major speciations. As more and more compelling direct evidence for inter-species and species-to-species evolution has been gathered, creationists have redefined their understanding of what amounts to "created kinds", and have continued to insist that more dramatic demonstrations of evolution be experimentally produced. One version of this objection is "Were you there?", popularized by young Earth creationist Ken Ham. It argues that because no one except God could directly observe events in the distant past, scientific claims are just speculation or "story-telling". DNA sequences of the genomes of organisms allow an independent test of their predicted relationships, since species which diverged more recently will be more closely related genetically than species which are more distantly related; such phylogenetic trees show a hierarchical organization within the tree of life, as predicted by common descent. In fields such as astrophysics or meteorology, where direct observation or laboratory experiments are difficult or impossible, the scientific method instead