To describe apparently related fossil forms as members of different species we apply the kinds of differences apparent between comparable extant species. Some workers prefer to work only with differences that justify generic names. This degree of caution is not always necessary, but it is more desirable than the reverse case in which every little difference between two fossils is used as an excuse for describing new species. The necessary precautions come down to an awareness of the natural variation that occurs in any species, a sense of the genetic basis for such variation, and then a cautious usage of species designations. In this way we can quite confidently combine our data on extinct and extant forms and expand the data base used for evolutionary studies.

The relative positions of fossils: Chronology. The most informative aspect of the fossil record is that it allows us to perceive evolutionary history directly. This can happen, however, only when we have determined the relative ages of fossils correctly. The basic rule is that the oldest fossils are toward the bottom of the pile. The pile we refer to is the bedded sequence of fossils. The oldest fossil beds are laid down first, the next oldest on top of them, and so on up to the surface where today's forms are living. Ideally, then, the evolutionary sequence is simply read from bottom to top. But things are rarely ideal. Two phenomena, at least, can confuse the ideal state of affairs. One of these is that fossil beds can be moved. Geological folding and uplift, which can obscure an otherwise quite readable story, occur. Folding often reorients layers or strata of fossil beds relative to each other and uplift in one areacan raise a bed that was at the bottom, and quite old, in another area. Another troublesome phenomenon is the loss of strata or beds. Erosion can remove millions of years of sedimented history and thus produce gaps in the fossil record. Such gaps, depending on the amount of discontinuity they cause, can pose genuine problems in our understanding of how one form evolves into another.

The answer to the foregoing largely depends on common sense as well as the competence of the geologist and the paleontologist. Common sense tells us to go slow in interpreting sequences from fossil beds until the geologist can determine the degree of folding, uplift, and erosion. The paleontologist can then compare the results from one stratum with those from a comparable stratum elsewhere and thus carefully reconstruct historical sequences. Ultimately, when we deal with history, we look for absolute measures of time, not just relative ones. We want to know precisely how long ago a certain event occurred and not just that it occurred earlier or later than other events. The most informative method in this regard is an analysis that depends on the amount of specific radioisotopes present in a fossil bed or a fossil.