THEORY OF INSTINCTS 199 



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 tempera- 

 ture or by lowering the concentration of the sea- 

 water. Hence these instincts must depend upon 

 such reversible changes in the material of the proto- 

 plasm 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 tempera- 

 ture 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 tempera- 

 ture 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 Ueber- 

 einstimmung mit dem Heliotropismus der Pflanzen. Wurzburg 

 1890. 



2. GROOM and LOEB. Der Heliotropismus der Nauplien von 

 Balanus perforatus iind die periodischen Tiefenwanderungen pelagis- 

 cher Thiere. Biologisches Centralblatt, Bd. x., 1890. 



3. LOEB, J. Ueber den Instinct und Willen der Thiere. 

 Pflugers Archiv, Bd. xlvii., p. 407, 1890. 



1 1 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. 



