urged against us that instincts should be explained historically and not physiologically or causally. It seems to me that living organisms are machines and that their reactions can only be explained according to the same principles which are used by the physicist. Our ultimate aim in the analysis of instincts is to find out by which physical and chemical properties of protoplasm they are determined. Of course the physicist finds it useful to illustrate the mechanism of complicated machines by the comparison with simpler or older machines of the same kind. We have made use of this same method and heuristic principle by utilis ing in this book the reactions of simpler forms for the analysis of more complicated forms. Even if we were in possession of a scientific phylogeny instead of the fairy tales that go by that name at present, it would not relieve us of the task of explaining the instincts on the basis of the physical and chemical qualities of protoplasm.

11. At first sight it may seem a hopeless task to find a connection between the instinctive actions of animals and the properties of their protoplasm. And yet the task is not so great if we choose the right method. This method, in my opinion, consists in varying the instincts of an animal at desire. If we succeed in this we are able to find out how the physical qualities of protoplasm may affect the instincts. I have tried this in one case. A number of marine animals (Copepods, larvæ of Polygordius) which go away from the light can be forced to go to the light in two

ways, first by lowering the temperature, and second, by increasing the concentration of the sea-water (whereby the cells of the animals lose water). This instinct can again be reversed by raising the temperature or by lowering the concentration of the scawater. Hence these instincts must depend upon such reversible changes in the material of the protoplasm as can be brought about by a loss of water or by a reduction of temperature. What these changes are can only be determined by further experiments. We find other instances where decrease in temperature has the same physiological effects as a loss of water. Plant-lice exist in wingless and in winged forms. We can at any time cause the growth of wings in the wingless forms by lowering the temperature or by letting the plant dry out (whereby the amount of water in the cells of plant-lice is reduced).1

BIBLIOGRAPHY.

1. LOEB, J. Der Heliotropismus der Thiere und seine Uebereinstimmung mit dem Heliotropismus der Pflanzen. Würzburg 1890.

2. GROOM and LOEB. Der Heliotropismus der Nauplien von Balanus perforatus und die periodischen Tiefenwanderungen pelagischer Thiere. Biologisches Centralblatt, Bd. x., 1890.

3. LOEB, J. Ueber den Instinct und Willen der Thiere. Pflüger's Archiv, Bd. xlvii., p. 407, 1890.

'I have found repeatedly that by the same conditions by which phenomena of growth and organisation can be controlled the instincts are controlled also. This indicates that there is a common basis for both classes of life phenomena. This common basis is the physical and chemical character of the mixture of substances which we call protoplasm.

4. LOEB, J. Ueber künstliche Umwandlung positiv heliotropischer Thiere in negativ heliotropische und umgekehrt. Pflüger's Archiv, Bd. liv., 1893.

5. LOEB, J. On Egg Structure and the Heredity of Instincts. The Monist, July, 1897.

6. GOLTZ, F. Ueber den Einfluss des Nervensystems auf die Vorgänge während der Schwangerschaft und des Geburtsaktes. Pflüger's Archiv, Bd. ix., 1874.

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CHAPTER XIV

THE CENTRAL NERVOUS SYSTEM AND

HEREDITY

1. The question as to how far the central nervous system comes into consideration for the processes of heredity is of great importance in educational problems. If we could hope that, as a result of the activity of a generation, its descendants would be born. with a talent for this special activity, there would be a fertile field for the improvement of the human race. In order to decide this question, we must first turn our attention to those peculiarities which we know to be hereditary-namely, the form of the body and the instincts. The analysis of the instincts given in the previous chapter places us in a position to answer the question as to how they can be transmitted through the egg. All hereditary qualities of form, instincts, and reflexes must be transmitted through the sexual cells. The difficulty that appears is this: How can the sexual cells, which only represent a liquid mass enclosed in solid membranes, be the bearers of such apparently complicated structures as the forms that originate from them with their instincts and

reflexes? Either the apparent simplicity of the structure of the egg is only an illusion, and in reality the structure of the egg is no less complicated than the full-grown animal, or the sum of the elements which we call the form and instincts of the full-grown animal is only the resultant of a few simpler elements which can readily be transmitted through the egg without its possessing a complicated structure. The discussion of the mechanics of instincts in the last chapter shows the latter to be the case. Let us consider those instincts that depend on heliotropic reactions for instance, the flying of the moth into the flame. This instinct is unequivocally determined, first, by the presence of a substance in the surface of the animal which is sensitive to light, and second, by the symmetrical structure of the animal. For the transmission of a substance which is sensitive to light through the egg no complicated mysterious structure is necessary. Neither is a complicated structure necessary for the egg in order that it may transmit the symmetrical relations of the animal.

For the inheritance of form the conditions are not very different. The egg is not the bearer of the form of the full-grown animal, but of certain chemical substances, especially of ferments. According to the stereochemical configuration of the latter, the products of assimilation, and with these the materials of the body, turn out differently. The process of development is not only a morphological but a chemical differentiation, and new combinations of substances are