Darwin's Theory of Evolution as spelt out in The Origin of Species assumes that all living creatures differ in some small way from their parents and that, if two identical populations are placed in isolation, one from the other, in different environments, Natural Selection will, by infinitesimally small changes in different directions, transmute the two populations into two different species. Thus evolution is seen as a steady state process, which is taking place all the time. The fossil record, however, shows that species appear fully evolved, radiate and become extinct, usually to be replaced by a new and similar species which also appears fully evolved. For a long time these facts were explained away as gaps in the fossil record and missing links were assumed between one species and the one which replaced it. The punctuationists argue that evolution did occur, as the fossil record suggested, in a series of jumps. It is my own personal belief that, unless a population was placed under some kind of pressure to evolve, there is no good reason why it should be subject to speciation. This does not imply that no changes took place; they could be taking place at the level of genes to produce new blueprints for change should the pressure to do so ever come into play.

Dr. Tom Kemp of Oxford's University Museum 1 has just proposed a model to account for the jump phenomena as applied to the Synapsida, the mammal- like reptiles. To summarise his model he assumes that a relatively small population is isolated from the main species and is subjected to pressures which lead to speciation. A mass extinction wipes out the main species and the new species moves into the vacant ecosystem to replace it. This model certainly appears to explain the facts in the case of Synapsida.

The geological record in Caithness does not show many gaps. The fossil record is, however, punctuated by barren interludes, which are short in terms of geological time. These short breaks can be explained in terms of the cyclic nature of the sediments and these cycles may, in turn, be due to a climatic cycle such as can be seen to be taking place currently in the playa lake environments of the western United States2. The largest of these breaks in terms of thickness of sediment laid down in the Caithness succession is the so-called Red Beds, which occur to the south of Wick. This interlude is not known to have been responsible for any species extinction however.

The figures show the succession of beds in the Caithness Old Red Sandstone, together with some of the genera and species which occur in them. A stepwise replacement of one species by another, the so-called species turnover, and a similar replacement of one genus by another can be clearly seen. Using the general model one can assume a mass extinction of all the Emsian species of the Basement beds. The osteolepids, the fish that became amphibia, show the clearest pattern. Thursius macrolepidotus can be assumed to have speciated during the late Emsian or early Eifelian to become extinct in the Wick Beds. Further speciation of this genus about the junction between the Eifelian and the Givetian could have given rise to Thurius pholidotus in the Spittal Beds to become extinct in the Mey Beds. Thursius is, however, replaced by Osteolepis macrolepidotus in the Achanarras Horizon during which period it also became extinct. It is presumed that Osteolepis panderi speciated during the Achanarras Horizon to become extinct during the Spittal Beds. Gyroptychius agassizi presumably speciated during the Wick Beds to appear and become extinct during the Achanarras horizon. Gyroptychius milleri speciated during the Achanarras Horizon to become extinct in the Spittal Beds. Tristichopterus presumably speciated during the Mey Beds to become extinct in the John o' Groats Sandstones. Eusthenopteron presumably speciated in the lower part of the Upper Old Red to give rise in turn to the early amphibia such as Ichthyostega.

The remainder of the Palaeozoic fishes do not show species but rather genus turnover, which suggests more rapid evolution. The Holoptychids have a succession from Porolepis in the Basement Beds to Glyptolepis which has a range from the Achanarras Horizon to the Mey Beds and, finally, to Holoptychius of the Dunnet Head Sandstones. From here the evolutionary line leads to the Coelocanths and a dead end. The Coccosteid Arthrodira are represented by Coccosteus in the Achanarras Horizon, Dickosteus in the Spittal Beds, Millerosteus in the Mey Beds and, finally, Watsonosteus in the John o' Groats Sandstones. The evolutionary line led to aberrant forms and, finally, to extinction. The Dipnoi do not show genus turnover since Dipterus and the later Pentlandia overlap. These fishes were, probably, herbivores and thus not under the same pressures to speciate. Their descendants are the modern lungfishes.

Whether or not the model of the Punctuation Theory is correct, the fossil record in Caithness does appear to proceed by a series of jumps. The weakness of the model lies in the assumption that a small population becomes isolated and then reunited after speciation with the original environment. Mass extinctions are easier to appreciate since one only needs a climatic cycle to wipe out a population and there is ample evidence for this in the Old Red Sandstone of Caithness, since fish beds are inter- spaced with barren beds, the barren beds predominating. It should be recognised, however, that a mass extinction due to a climatic deterioration does not necessarily imply the mass extinction of a species, since succeeding fish beds often contain the same fish fauna. The main body of the population must have survived the drought elsewhere only to recolonise the Orcadian Ouvette once conditions were again satisfactory. This clearly implies that populations were isolated and reunited many times as the model requires. The argument rests to some extent as to whether the genera and species are true genera and species and not merely convenient systems of classification drawn up by taxonomists, and that is something we are never likely to be able to ascertain to everybody's satisfaction.