Phylogeny and the Dimension of Time

Abstract

The paleontological record is the ultimate test of phylogeny. This categorical statement can be defended upon two counts; first on the ground that the paleontological record is the only direct evidence of phylogeny, and second, that this record gives us the fourth dimension of time in evolution. Actually these two arguments are interrelated, and the one supports the other. Phylogeny is the expression of evolutionary relationships between organisms, and in a final analysis the only absolute test of these relationships is the evolutionary record through time. Truly the dimension of time looms large in phylogeny. This fact is much appreciated by the paleontologist, but perhaps is not so fully and constantly within the consciousness of the biologist. The student who works with recent organisms may admit the importance of time in evolution, but since his work and his thoughts are confined to the contemporary scene it is difficult for him to be so completely aware of the fourth dimension of time as is his colleague who works with fossils. This difference in view point between the neontologist and the paleontologist is perfectly natural. So it is that the biologist not infrequently constructs so-called phylogenies made up of graded series of modern organisms. Except in rare cases these are not true phylogenies, rather they are sequences or gradients showing step by step changes between subspecies or species, or perhaps genera, these steps commonly being based upon morphological characters. They may, however, involve other attributes as well-for example, behavior patterns. To the paleontologist such arrangements of modern organisms are hardly to be viewed as phylogenies; he sees them as lateral series, lacking the vertical time component that to him is so essential for the phylogenetic interpretation of evolution. Of course it may be argued that the vertical sequences of extinct organisms, arranged according to their age by the paleontologist, are not true phylogenies either, since fossils of one age cannot be unquestionably established as the actual ancestors of those belonging to a later age. Yet however true this may be on philosophical grounds, there can be no doubt but that in realistic terms documented paleontological phylogenies approach very closely the ideal of a direct ancestor-descendant record of relationships. It may, for instance, be difficult to say that any particular species of Miocene horse is the direct ancestor of a Pliocene species, but