The science of life : fully illustrated in tone and line and including many diagrams

BOOK 38

And they link reptiles with mammals in yet another way. One of the most important differences between existing mammals and all other vertebrates is the fact that in mammals the lower jaw consists of but one bone on each side, instead of several distinct bones stuck together. At the same time, there are three little bones in the mammal’s

A vegetable missing link.

Fig. 130. are expanded, the others still curled up. of a frond bearing seeds is shown enlarged ;

Scott. Courtesy of Macmillan & Co., Lid.)

middle-ear, transmitting the vibrations of

its ear-drum to its organ of hearing, while in all other land vertebrates there is but one. It had been suspected for a long time that these two extra bones, the so-called “ hammer” and “anvil,” correspond to the two bones which in a reptile make the

hinge-joint of upper with lower jaw ; for they are

in a mammal embryo before

QTd.

THE SCIENCE OF LIFE

Lyginopteris, one of the Pteridosperms or Seed-ferns, from the Carboniferous Period (III E).

A plant is shown bearing seeds on one of its fronds.

Three fronds Below, on the left, a bit on the right, a young, unexpanded frond. The reconstruction was made by Miss 7. Robertson. (From “ Extinct Plants and Problems of Evolution,” by Dr. D. H.

CHAPTER 2

built into the machinery of the ear they are actually nipped off from this very region of the developing jaws. And the tympanic bone, on which the ear-drum is stretched, can similarly be homologized with another of the multiple bones of the reptile’s lower jaw. Now, in the Theromorphs most of the steps in this process can actually be traced. In the earliest fossils, the arrangement is like that of other reptiles. But as we go forward in time, the, two hinging bones of the jaws gradually release themselves from this duty, grow smaller and are to bé found in the region of the ear ; from this condition, only a small step would be needed to convert them into ear-ossicles. Meanwhile the future tympanic bone also was becoming less concerned with biting and more with hearing (Fig. 129). Archeopteryx had just raised itself to bird status ; the Theromorphs were still reptiles but were on the verge of climbing out into a higher stratum of biological society.

If we want an example of such a transition among the invertebrates, we can go to the brittle-stars. These Echinoderms are very like starfishes, but their central body or disc is much more sharply marked off from their wriggling arms. In all living brittlestars the grooves along the lower surface of the arms, so prominent in starfishes with the tube-feet arranged along them, have disappeared below the surface, roofed over to form tunnels ; further, the main skeleton of each arm is a chain of little ossicles, beautifully jointed together. But in the Silurian (III GC) and Devonian (III D) Periods there existed obvious brittle-stars which possessed open arm-grooves and armskeletons of much less elaborate construction: in both these ways they link the existing brittle-stars with the true starfishes. They were starfishes becoming brittle.

There are plant missing links as well as animal ones. The Pteridosperms, or “‘seedferns? which flourished during the latter half of the Paleozoic Era (III), are linking types of rather a different description. They appear not to provide a direct connection between seed-bearing plants and the true ferns, seedless and spore-producing ; but rather to be a side-branch (like the Hipparion branch of the horse stock),