The Succession of Faunas

Phylogenies.

The first step in the construction of a phy logeny is to determine that certain animals whose remains are known from rocks of different ages are in fact related to one another. This can only be done by finding that a common plan of organisation runs through all of them and that they are held together and distinguished from associated and related forms by the possession in common of certain characters, which persist unchanged or little altered from the earlier to the later animals. Such characters, called palaeotelic, are often inconspicuous and difficult to discover. Amongst vertebrates they are to be found in such regions as the brain-case and ankle-joint, which are little exposed to adaptive changes. The certainty with which it can be shown that a series of animals of different ages are in fact related to one another, depends entirely on the number of palaeo telic characters which can be discovered in their structure. Thus far greater certainty can be attained in the case of animals with complex skeleton, such as vertebrates and echinoids, than in such creatures as Mollusca and graptolites, whose skeleton is simple.

When by this method a series of allied animals has been sorted out from amongst its contemporaries, the earliest of them should be compared with the most recent. Such a comparison will bring out differences capable of being recorded. Those members of the series which are of intermediate age must then be compared with the beginning and the end, and the nature of the differences which they display considered with respect to the changes which sepa rate the first and last of the series. If the series has been cor rectly discriminated it will usually be found that all the forms fall into order, each differing from that which precedes it in the same ways as its successor differs from it.

Micraster.

The best case of such an evolutionary series, illus trating the origin of species, is that presented by the genus Mi craster, a heart-urchin, found commonly in the Middle and Upper Chalk of England; this case was described by Dr. A. W. Rowe. Dr. Rowe collected 2,000 examples of echinoids from the chalk of the south of England, the geological horizon of every individual being accurately recorded. He then measured all these speci mens and, having taken microphotographs of the details of their structure and compared the whole group of specimens from each zone with the other similar groups, decided that 18 different characters could be recognised in each test, and for each of these characters he compared the whole group of forms from each zone with that from every other zone. As a result of this comparison he

was able to show that for most of the i8 characters, there was a change from the earliest to the most recent assemblages and that the Micrasters from each of the five zones he considered, differed from their predecessors as their successors differed from them.

For example, the character whose evolutionary change is perhaps most readily illustrated is that presented by the paired ambulacra: each one of these structures consists of a strip of the surface of the skeleton of the animal built up by two rows of small plates rigidly attached to each other by their edges and interlocking down the centre of the ambulacrum in a zig-zag line. Each plate is pierced by two small holes which are connected with the tube feet, freely movable structures with a sucker on their outer end.

Between the inner rows of pores there is an area within which great changes take place with time. In the earliest forms this interporiferous area is smooth and the whole ambulacrum forms a deep rounded valley. In the next higher zone the interporiferous area is divided up by fine incised lines along the margin of the plates and its surface is no longer quite smooth but bears a very faint granulation. In later zones the depth of the incisions between the plates increases and their inner ends become somewhat raised, the granulation is more pronounced and the whole ambulacrum, although still a deep groove, is no longer smoothly rounded. In the next stage the inner ends of the plates are so thickened as to form a V-shaped groove running continuously down the mid-line of the area. Finally the ambulacrum comes to form only a very slight depression with a deep gutter along the mid-line of the nar row interporiferous area, and the surface of each plate above and between the pores is covered with a dense granulation. These changes take place steadily with time, but all individuals living at any one period were not in the same stage of evolution. For example, the smooth type of area is found only in the two upper zones of the Middle Chalk and in the lower zone of the Upper Chalk, but whereas it is the only form found in the Middle Chalk, it occurs in 20% of Micrasters from the Upper Chalk zone. The next type in which the plates are marked out by incised lines occurs very rarely in the top zone of the Middle Chalk, and in 44% of Micrasters from the lowest zone of the Upper Chalk. In this zone 30% of the specimens are of the type in which the inner ends of the plates are thickened whilst in the succeeding zone 50% belong to this stage.