ate with the changes in the state of this gland. Our normal skin and features are thus dependent upon the integrity of this internal organ.

The mechanism of correlation between two such organs as the thyroid and the skin has already been somewhat worked out. It is natural to suspect that this correlation is nervous, for both thyroid and skin are supplied with an abundance of nerves coming from a common central organ. But the fact that the symptoms already described as the result of the removal of the thyroid can be checked and even made to disappear by grafting into the animal that has lost its thyroid, a part of a living gland from another animal, shows conclusively that the nervous system is not concerned. The further observation that animals devoid of thyroids may be kept in normal condition by injecting thyroid juice into them or even by feeding them with fresh thyroid glands from other animals, has suggested the idea that this gland produces a substance which makes its way into the blood and is thus carried to those parts of the body where it is needed. It is through this substance that the skin is influenced in that in the absence of this material the skin suffers serious change. The mechanism of correlation between the thyroid gland and the skin, then, consists in a substance produced by the gland and carried in the fluids of the body to various organs, including the skin, whose growth and appearance is thereby modified.

Similar observations have led to a like conclusion concerning the action of the suprarenal bodies and the hypophysis. These organs, like the thyroid, produce substances that make their way into the fluids of the body and influence its structure and action in so profound a way that they are absolutely essential to its continued existence. In the case of the suprarenal bodies the active substance has been isolated and is known as adrenalin. Since these internal secretions have the power of calling forth or exciting very marked changes in the body, they have been given the general name of hormones. It would,

however, probably be a mistake to regard the production of these hormones as limited to a few organs such as the thyroids, suprarenal bodies, etc. The most recent work in this field points to the conclusion that all active organs of the body, nerve centers, muscles, glands, etc., produce hormones which in the blood probably exert extensive influences on the parts with which they come in contact, and examples of this kind are being rapidly discovered. It was formerly supposed that the secretion of the pancreatic juice, which is poured into the small intestine when the partly digested food from the stomach reaches that organ, was dependent upon a nervous signal given to the pancreas from the intestine, but it is now well established, through the brilliant work of Bayliss and Starling, that the action of the acid food on the walls of the intestine produces a hormone, called secretin, which when carried in the blood to the pancreas will cause that organ to secrete. The evidence of this lies in the fact that when a small amount of secretin is injected directly into the blood stream of a mammal, the pancreas, whose nerve supply may have been cut off, will begin to secrete without the presence of food in the intestine. Still more remarkable is the correlation between the mammary glands and the embryo in mammals. It is well known that as the time for the birth of a mammal approaches, the mammary glands of the parent grow in size and structural changes appear preparatory to the secretion of milk. This correlation between the growth of the embryo and the growth of the mammary glands can not depend upon nervous coordination, for the nerves of the embryo have no connection with those of the maternal body. The correlation depends upon a substance, a hormone, produced in the body of the embryo and transmitted to the blood of the mother, whereupon it so influences the mammary glands as to start their growth. The evidence for this lies in the fact that if the extracted juice of a rabbit embryo is injected periodically into the circulation of a virgin female rabbit, her mammary glands can be in

duced to take on the growth characteristic of the early stages of pregnancy though she is absolutely without

young.

Another important set of bodily correlations are those that exist between the reproductive glands and the secondary sexual organs such as the comb, hackles and spurs of the common male fowl. It is well known that if the genital glands of a young male fowl are removed before it has attained maturity, it will fail to perfect its secondary sexual organs and the usual external evidences of maleness may be absent. But if, as Shattock and Seligmann have shown, a small piece of a male gland is grafted into a young castrated male the comb, hackles, and spurs may develop as in a normal bird. It is, therefore, highly probable that the reproductive glands, like the ductless glands, produce hormones by which the development of the secondary sexual organs is determined.

Not only are hormones produced in the adult body, but they are very probably formed during development. Such at least seems to be the condition in the correlated growth of the vertebrate eye and its lens. As is well known, the eyeball in the vertebrate is formed around an outgrowth from the brain; the lens is developed from the skin in such a position as to fit the forming ball. This interesting correlation in position between the external lens and the deep-seated eyeball has been made clear by Lewis who has shown that when the forming eyeball of a given species of frog is covered by grafting over it skin from the abdominal region of even another species of frog, this foreign abdominal skin will begin to form a lens in an appropriate position for the underlying eyeball. Apparently the eyeball gives out a substance, a hormone, that so influences the adjacent skin that, irrespective of its source within certain limits, it forms a lens. Thus embryonic correlations may also depend upon hormones.

These numerous examples show that many organs of the body produce hormones that profoundly affect the

form and structure of many other organs, external as well as internal. And further that these hormones are in some cases absolutely essential to the continuance of life. In short we must consider the interior of every organism as exhibiting an environment to which every organ probably contributes and by which every organ is more or less influenced. The hormones of this environment are the mechanisms of correlation and by means of them one organ influences another. It is no longer necessary to describe organic correlation as an unknown law of growth. It is the dependence of one organ on another through the hormones that the influencing organ produces.

Granting this condition, it follows that natural selection may well be conceived to modify an internal hormoneproducing organ, if this organ is of vital significance, and incidentally thus to establish a new internal environment that would so influence the form and external configuration of a given organism that it would be called a new species and yet none of the new external features by which this organism would be described might show the least usefulness.

RECENT ADVANCES IN THE STUDY OF

VASCULAR ANATOMY1

I. VASCULAR ANATOMY AND THE REPRODUCTIVE

STRUCTURES

PROFESSOR JOHN M. COULTER

UNIVERSITY OF CHICAGO

It is perhaps unfortunate that the names applied to the great divisions of botanical investigation shift in their meaning from time to time, but it is inevitable. The content of a subject shifts with the men who put content into it. The morphology of to-day is not the morphology of half a century ago, either in its content or motive; or rather there are several conceptions of morphology existing side by side, some as an inheritance, and others as acquired characters. The older conception of morphology, presented, for example, in the model textbooks of Asa Gray, is one thing; and that introduced by the work of Hofmeister, which very slowly made its way into this country, is a very different thing.

This more recent morphology adds to the old knowledge of structures the relation of these structures in a scheme of phylogeny. Its importance lies not so much in the fact that it solves the perennial problem of phylogeny, as in the fact that it calls for the selection and comparison of structures throughout the plant kingdom. It takes the enormous débris of material that has accumulated and sifts it, passing over the trivial, emphasizing the important, and building up the body of knowledge into a structure that has some form. As knowledge advances, the trivial of yesterday may become the important to-day, and vice versa; but the building of a structure, upon any plan, is work of a higher order

1 Papers prepared by request of the Council and read at the Baltimore meeting of the Botanical Society of America.