THE MECHANISTIC CONCEPTION OF LIFE 19 



8. The Contents of Life 



The contents of life from the cradle to the bier are wishes and hopes, 

 efforts and struggles and unfortunately also disappointments and suf- 

 fering. And this inner life should be amenable to a physico-chemical 

 analysis? In spite of the gap which separates us to-day from such an 

 aim I believe that it is attainable. As long as a life phenomenon has 

 not yet found a physico-chemical explanation it usually appears inex- 

 plicable. If the veil is once lifted we are always surprised that we did 

 not guess from the first what was behind it. 



That in the case of our inner life a physico-chemical explanation is 

 not beyond the realm of possibility is proved by the fact that it is 

 already possible for us to explain cases of simple manifestations of 

 animal instinct and will on a physico-chemical basis ; namely, the phe- 

 nomena which I have discussed in former papers under the name of 

 animal tropisms. As the most simple example we may mention the 

 tendency of certain animals to fly or creep to the light. We are deal- 

 ing in this case with the manifestation of an instinct or impulse which 

 the animals can not resist. It appears as if this blind instinct which 

 these animals must follow, although it may cost them their life might 

 be explained by the same law of Bunsen and Eoscoe, which explains 

 the photo-chemical effects in inanimate nature. This law states that 

 within wide limits the photo-chemical effect equals the product of the 

 intensity of light into the duration of illumination. It is not possible 

 to enter here into all the details of the reactions of these animals to 

 light, we only wish to point out in which way the light instinct of the 

 animals may possibly be connected with the Bunsen-Roscoe law. 



The positively heliotropic animals — i. e., the animals which go 

 instinctively to a source of light — have in their eyes (and occasionally 

 also in their skin) photosensitive substances which undergo chemical 

 alterations by light. The products formed in this process influence the 

 contraction of the muscles — mostly indirectly, through the central 

 nervous system. If the animal is illuminated on one side only the 

 mass of photochemical reaction products formed on that side in the 

 unit of time is greater than on the opposite side. Consequently the 

 development of energy in the symmetrical muscles on both sides of the 

 body becomes unequal. As soon as the difference in the masses of the 

 photochemical reaction products on both sides of the animal reaches a 

 certain value the animal, as soon as it moves, is automatically forced 

 to turn towards one side. As soon as it has turned so far that its 

 plane of symmetry is in the direction of the rays, the symmetrical spots 

 of its surface are struck by the light at the same angle and in this case 

 the intensity of light and consequently the velocity of reaction of the 

 photochemical processes on both sides of the animal become equal. 

 There is no more reason for the animal to deviate from the motion in a 



