cated and highly developed reproductive organs. By markedly higher forms they certainly grow simpler.

And here we must notice certain general considerations. We found that reproduction in the amœba could be defined as growth beyond the limit normal to the individual. This form of growth benefits especially the species. The needs and expenses of the individual will therefore first be met and then the balance be devoted to reproduction. Now the income of the animal is proportional to its surface, its expense to its mass and activity. And the ratio of surface to mass is most favorable in the smallest animals.* Hence, smaller animals, as a rule, increase faster than larger ones; and this is only one illustration of the fact that great size in an animal is anything but an unmixed advantage to its possessor. But muscles and nerves are the most expensive systems; here most of the food is burned up. Hence energetic animals have a small balance remaining. Now the turbellarian is small and sluggish, with a fair digestive system. With a great amount of nutriment at its disposal the reproductive system came rapidly to a high development, and relatively to other organs stands higher than it almost ever will again.

.

It is only fair to state that good authorities hold that so primitive an animal could not originally have had so highly developed a system, and that this characteristic must be acquired, not ancestral.

That certain portions of it may be later developments may be not only possible but probable. But anyone who has carefully studied the different groups of worms, will, I think, readily grant that in the stage

* Cf. p. 35.

of these flat worms reproduction was the dominant function, which had most nearly attained its possible height of development. From this time on the muscular and nervous systems were to claim an ever-increasing share of the nutriment, and the balance for reproduction is to grow smaller.

At the close of this lecture I wish to describe very briefly a hypothetical form. It no longer exists; perhaps it never did. But many facts of embryology and comparative anatomy point to such a form as a very possible ancestor of all forms higher than flat worms, viz., mollusks, arthropods, and vertebrates.

It was probably rather long and cylindrical, resembling a small and short earthworm in shape. The skin may have been much like that of turbellaria. Within this the muscles run in only two directionslongitudinally and transversely. Between these and the intestine is a cavity-the perivisceral cavity—like that of our own bodies, but filled with a nutritive fluid like our lymph. This cavity seems to have developed by the expansion and cutting off of the paired lateral outgrowths of the digestive system of some old flat worm. But other modes of development are quite possible. The intestine has now an anal opening at or near the rear end of the body. The food moves only from front to rear, and reaches each part always in a certain condition. Digestion proper and absorption have been distributed to different cells, and the work is better done. Three portions can be readily distinguished fore-intestine with the mouth, mid-intestine, as the seat of digestion and absorption, and hindintestine, or rectum, with the anal opening. The front and hind-intestine are lined with infolded outer skin.

The nervous system consists of a supra-œsophageal ganglion with four posterior nerve-cords—one dorsal, two lateral, and one (or perhaps two) ventral. There were probably also remains of the old plexus, but this is fast disappearing. The excretory system consists of a pair of tubes discharging through the sides of the body-wall, and having each a ciliated, funnel-shaped opening in the perivisceral cavity. These have received the name of nephridia. Through these also the eggs and spermatozoa are discharged. The reproductive organs are modified patches of the peritoneum, or lining of the perivisceral cavity.

The number of muscles or muscular layers has been reduced in this animal. But such a reduction in the number of like parts in any animal is a sign of progress. And the longitudinal muscles have increased in size and strength, and the animal moves by writhing. Such a worm has the general plan of the body of the higher forms fairly well, though rudely, sketched. Many improvements will come, and details be added. But the rudiments of the trunk of even our own bodies are already visible. Head, in any proper sense of the term, and skeleton are still lacking; they remain to be developed.

And yet, taking the most hopeful view possible concerning the animal kingdom, its prospects of attaining anything very lofty seem at this point poor. Its highest representative is a headless trunk, without skeleton or legs. It has no brain in any proper sense of the word, its sense-organs are feeble; it moves by writhing. Its life is devoted to digestion and reproduction. Whatever higher organs it has are subsidiary to these lower functions. And yet it has taken

ages on ages to develop this much. If this is the highest visible result of ages on ages of development, what hope is there for the future? Can such a thing be the ancestor of a thinking, moral, religious person, like man? "That is not first which is spiritual, but that which is natural (animal, sensuous); and afterward that which is spiritual." First, in order of time, must come the body, and then the mind and spirit shall be enthroned in it. The little knot of nervous material which forms the supra-oesophageal ganglion is so small that it might easily escape our notice; but it is the promise of an infinite future. The atom of nervous power shall increase until it subdues and dominates the whole mass.

CHAPTER III

WORMS TO VERTEBRATES: SKELETON AND HEAD

IN tracing the genealogy of any American family it is often difficult or impossible to say whether a certain branch is descended from John Oldworthy or his cousin or second cousin. In the latter cases to find the common ancestor we must go back to the grandfather or great-grandfather. The same difficulty, but greatly enhanced, meets us when we try to make a genealogical tree of the animal kingdom. Thus it seems altogether probable that all higher forms are descended from an ancestor of the same general structure and grade of organization as the turbellaria, although probably free swimming, and hence with somewhat different form and development, especially of the muscular system. It seems to me altogether probable that all, except possibly Mollusca, are descended from a common ancestor closely resembling the schematic worm last described. Some would, however, maintain that they diverged rather earlier than even the turbellaria; others after the schematic worm, if such ever existed. As far as our argument is concerned it makes little difference which of these views we adopt.

From our turbellaria, or possibly from some even more primitive ancestor, many lines diverged. And this was to be expected. The colenterata, as we saw in hydra, had developed rude digestive and reproduc