injurious effects of strong light. Another striking characteristic in such reflexes has also been emphasised. The movements which are produced are so well planned and coördinated that it seems as though some intelligence were at work either in devising or in carrying them out. The fact, however, that a decapitated frog will brush a drop of acetic acid from its skin, suggests that some other explanation is necessary. A prominent psychologist has maintained that reflexes are to be considered as the mechanical effects of acts of volition of past generations. The ganglion-cell seems the only place where such mechanical effects could be stored up. fore been considered the most essential element of It has therethe reflex mechanism, the nerve-fibres being regarded, and probably correctly, merely as conductors.

Both the authors who emphasise the purposefulness of the reflex act, and those who see in it only a physical process, have invariably looked upon the ganglion-cell as the principal bearer of the structures for the complex coördinated movements in reflex action.

I should have been as little inclined as any other physiologist to doubt the correctness of this concepion had not the establishment of the identity of the hactions of animals and plants to light proved the tenability of this view and at the same time offered 'ifferent conception of reflexes. The flight of coth into the flame is a typical reflex process. Cight stimulates the peripheral sense organs, the

possess

stimulus passes to the central nervous system, and from there to the muscles of the wings, and the moth is caused to fly into the flame. This reflex process agrees in every point with the heliotropic effects of light on plant organs. Since plants possess no nerves, this identity of animal with plant heliotropism can offer but one inference - these heliotropic effects must depend upon conditions which are common to both animals and plants. At the end of my book on heliotropism I expressed this view in the following words: 'We have seen that, in the case of animals which possess nerves, the movements of orientation toward light are governed by exactly the same external conditions, and depend in the same way upon the external form of the body, as in the case of plants which no nerves. These heliotropic phenomena, consequently, cannot depend upon specific qualities of the central nervous system (1)." On the other hand, the objection has been raised that destruction of the ganglion-cells interrupts the reflex process. This argument, however, is not sound, for the nervous reflex arc in higher animals forms the only protoplasmic bridge between the sensory organs of the surface of the body and the muscles. If we destroy the gan glion-cells or the central nervous system, we interrupn the continuity of the protoplasmic conduction betweex the surface of the body and the muscles, and a ref the is no longer possible. Since the axis-cylinders of than nerves and the ganglion-cells are nothing more r eeking protoplasmic formations, we are justified in s

in them only general protoplasmic qualities, unless we find that the phenomena cannot be explained by means of the latter alone.

2. A further objection has been raised, that although these reflexes occur in plants possessing no nervous system, yet in animals where ganglion-cells are present the very existence of ganglion-cells necessitates the presence in them of special reflex mechanisms. It was therefore necessary to find out if there were not animals in which coördinated reflexes still continued to exist after the destruction of the central nervous system. Such a phenomenon could be expected only in forms in which a direct transmission of stimuli from the skin to the muscle is possible, in addition to the transmission through the reflex arc. This is the case, for instance, in worms and in Ascidians. I succeeded in demonstrating in Ciona intestinalis that the complicated reflexes still continue after removal of the central nervous system (2).

A study, then, of comparative physiology brings out the fact that irritability and conductibility are the only qualities essential to reflexes, and these are both common qualities of all protoplasm. The irritable structures at the surface of the body, and the arrangement of the muscles, determine the character of the reflex act. The assumption that the central nervous system or the ganglion-cells are the bearers of reflex mechanisms cannot hold. But have we now to conclude that the nerves are superfluous and a waste? Certainly not. Their value lies in the fact that they

are quicker and more sensitive conductors than undifferentiated protoplasm. Because of these qualities of the nerves, an animal is better able to adapt itself to changing conditions than it possibly could if it had no nerves. Such power of adaptation is absolutely necessary for free animals.

3. While some authors explain all reflexes on a psychical basis, the majority of investigators explain in this way only a certain group of reflexes - the socalled instincts. Instincts are defined in various ways, but no matter how the definition is phrased the meaning seems to be that they are inherited reflexes so purposeful and so complicated in character that nothing short of intelligence and experience could have produced them. To this class of reflexes belongs the habit possessed by certain insects of laying their eggs on the material which the larva will afterwards require for food. When we consider that the female fly pays no attention to her eggs after laying them, we cannot cease to wonder at the seeming care which nature takes for the preservation of the species. How can the action of such an insect be determined if not by mysterious structures which can only be contained in the ganglion-cells? How can we explain the inheritance of such instincts if we believe it to be a fact that the ganglion-cells are only the conductors of stimuli? It was impossible either to develop a mechanics of instincts or to explain their inheritance in a simple way from the old standpoint, but our conception makes an explanation possible. Among the

elements which compose these complicated instincts, the tropisms (heliotropism, chemotropism, geotropism, stereotropism) play an important part. These tropisms are identical for animals and plants. The

explanation of them depends first upon the specific irritability of certain elements of the body-surface, and, second, upon the relations of symmetry of the body. Symmetrical elements at the surface of the body have the same irritability; unsymmetrical elements have a different irritability. Those nearer the oral pole possess an irritability greater than that of those near the aboral pole. These circumstances force an animal to orient itself toward a source of stimulation in such a way that symmetrical points on the surface of the body are stimulated equally. In this way the animals are led without will of their own either toward the source of the stimulus or away from it. Thus there remains nothing for the ganglioncell to do but to conduct the stimulus, and this may be accomplished by protoplasm in any form. For the inheritance of instincts it is only necessary that the egg contain certain substances - which will determine the different tropisms-and the conditions for producing bilateral symmetry of the embryo. The mystery with which the ganglion-cell has been surrounded has led not only to no definite insight into these processes, but has proved rather a hindrance in the attempt to find the explanation of them.

It is evident that there is no sharp line of demarcation between reflexes and instincts. We find that