In the turbellaria the dominant functions were digestion and reproduction, and their organs composed almost the whole body. Here only eleven segments at most are devoted to these functions, and nine in head and thorax to locomotion and brain. Head and thorax have increased steadily in importance, while the abdomen has decreased as steadily in number of segments. And the brain is increasing thus rapidly because there are now muscles and sense-organs of sufficient power to make such a brain of value. And this brain perceives not only objects and qualities, but invisible relations between these, and this is an advance amounting to a revolution. It remembers, and uses its recollections. It is capable of learning a little by experience and observation. The A, B, C of thinking was probably learned long before the insect's time, and the bee shows a fair amount of intelligence.

The line of development which the insect followed was comparatively easy and its course probably rapid. Certain crustacea, aquatic arthropoda, are among the oldest fossils, and it is possible that insects lived on the land before the first fish swam in the sea. They had fine structure and powers; and yet during the later geologic periods they have scarcely advanced a step, and are now apparently at a standstill. They ran splendidly for a time, and then fell out of the race. What hindered and stopped them?

One vital defect in their whole plan of organization is evident. The external skeleton is admirably suited to animals of small size, but only to these. In larger animals living on land it would have to be made so heavy as to be unwieldy and no longer economical. Their mode of breathing also is fitted only for animals

of small size having an external skeleton. Whatever may be our explanation the fact remains that insects are always small. This is in itself a disadvantage. Very small animals cannot keep up a constant high temperature unless the surrounding air is warm, for their radiating surface is too large in comparison with their heat-producing mass. At the first approach of even cool weather they become chilled and sluggish, and must hibernate or die. They are conformed to but a limited range of environment in temperature. But small size is, as a rule, accompanied by an even greater disadvantage. It seems to be almost always correlated with short life. Why this is so, or how, we do not know. There are exceptions; a crow lives as long as a man; or would, if allowed to. But, as a rule, the length of an animal's days is roughly proportional to the size of its body. And the insect is, as a rule, very short-lived. It lives for a few days or weeks, or even months, but rarely outlasts the year. It has time to learn but little by experience. The same experience must be passed, the same emergency arise and be met, over and over again during the lifetime of the same individual if the animal is to learn thereby. And intelligence is based upon experience. Hence insects can and do possess but a low grade of intelligence. But instinct is in many cases habit fixed by heredity and improved by selection. The rapid recurrence of successive generations was exceedingly favorable to the development of instincts, but very unfavorable to intelligence. Insects are instinctive, the highest vertebrates intelligent. The future can never belong to a tiny animal governed by instincts. Mollusks and insects have both failed to reach the goal; another

plan of structure than theirs must be sought if the animal kingdom is to have a future.

The future belonged to the vertebrate. To begin with less characteristic organs the digestive system is much like that of the annelid or schematic worm, but with greatly increased glandular and absorptive surfaces. The present mouth of nearly all vertebrates is probably not primitive. It is almost certainly one of the gill-slits of some old ancestor of fish, such as now are used to discharge the water which is used for respiration. The jaws are modified branchial arches or the cartilaginous or bony rods which in our present fish support the fringe of gills. These have formed a pair of exceedingly effective and powerful jaws. The reproductive system holds still to the old type and shows little if any improvement. The excretory organs, kidneys, are composed primitively of nephridial tubes like those of the schematic worm or annelid, but immensely increased in number, modified, and improved in certain very important particulars. The muscles in simplest forms are composed of heavy longitudinal bands, especially developed toward the dorsal surface of the body to the right and left of the axial skeleton. Locomotion was produced by lashing the tail right and left, as still in fish. There is improvement in all these organs, except perhaps the reproductive, but nothing very new or striking. The great improvement from this time on was not to be sought in the vegetative organs, or even directly to any great extent in muscles.

The new and characteristic organ was not the vertebral column, or series of vertebræ, or backbone, from which the kingdom has derived its name. This was a

F....

SS

later production. The primitive skeleton was the notochord, still appearing in the embryos of all vertebrates and persisting throughout life in fish. This is an elastic rod of cartilage, lying just beneath the spinal marrow or nerve-cord, which runs backward from the brain. The nerve-centres are therefore here all dorsal, and the notochord or skeleton lies between these and the digestive or alimentary canal. The skeleton of the clam or snail is purely protective and a hindrance to locomotion. That of ss the insect is almost purely locomotive, but external, that of the vertebrate purely locomotive and internal. It does not lie outside even of the nervous

P

M

F.-

--Ee

C

F...

10. CROSS-SECTION OF

Cs

AXIAL system, although this system especially required, and was

SKELETON OF PETROMYZON.

HERTWIG, FROM HIEDERSHEIM.

SS, skeletogenous layer; ob, ʊb, worthy of, protection. It does

dorsal and ventral processes of SS; C, notochord; Cs, sheath of notochord; Ee, elastic ex

tissue; M, spinal marrow; P, sheath of M.

not protect even the brain; the skull of vertebrates is an af

ternal layer of sheath; F, fatty ter-thought. It is almost the deepest seated of all organs. But lying in the central axis of the body it furnishes the very best possible attachment for muscles. Around this primitive notochord was a layer of connectile tissue which later gave rise to the vertebræ forming our backbone.

The nervous system on the dorsal surface of the notochord consists of the brain in the head and the spinal

marrow running down the back. The brain of all except the very lowest vertebrates consists of four portions: 1. The cerebrum, or cerebral lobes, or simply "forebrain," the seat of consciousness, thought, and will, and from which no nerves proceed. Whether the

primitive vertebrate had any cerebrum is still uncertain. 2. The mid-brain, which sends nerves to the eyes, and in this respect reminds us of the brain of insects. Its anterior portion appears from embryology to be very primitive. 3. The small brain, or cerebellum, which in all higher forms is the centre for co-ordination of the motions of the body. 4. The medulla, which controls especially the internal organs. The spinal marrow, or that portion of the nervous system which lies outside of the head, is at the same time a great nerve-trunk and a centre for reflex action of the muscles of the body. But the development of these distinct portions and the division of labor between them must have been a long and gradual process.

We have every reason to believe that here, as in insects, the head has been formed by annexation of segments from the rump and the fusion of their nervous matter with that of the brain. But here, instead of only three segments, from nine to fourteen have been fused in the head to furnish the material for the brain. Notochord and backbone may be the most striking and apparent characteristic of vertebrates, but their predominant characteristic is brain. On this system they lavished material, giving it from three to four times as much as any lower or earlier group had done. They very early set apart the cerebral lobes to be the commander-in-chief and centre of control for all other nerve-centres. To this all report, and from it all di