Mast, Loeb's Mechanistic Conception of Life. 



the term, but the evidence presented in favor of either of these 

 contentions is anything but convincing. 



Leaving now the question of the reduction of life processes 

 to mechanical principles, let us consider a few instances in which 

 Loeb's work seems to be open to criticism from other points 

 of view. 



In attempting to establish the idea "that vision is based on 

 the formation of an image on the brain" he makes use of two lines 

 of evidence, (a) results obtained in operations on the brain, and 

 (b) observation on the pattern adaptation in fishes. 



He says (p. 79) that the experiments of Munk on the brain 

 of dogs show that "there exists a projection of the retina on a 

 part of the cortex" designated as the visual sphere and he main- 

 tains in this connection, that these experiments have been confirmed 

 by Henschen and by Minkowski, but on page 35 he says, 

 referring to these same experiments: "Five years of experiments 

 with extirpations in the cerebral cortex proved to me without doubt 

 that Munk had become the victim of an error." His principal 

 source of evidence in support of the thesis in hand is however, he 

 maintains, found in Sumner's work on changes in the pattern of 

 the skin of certain fishes so as to continuously harmonise with the 

 background. 



Loeb holds that this work shows that the retinal image is 

 reproduced in the skin. He says (p. 81): "There exists, therefore, 

 a definite arrangement of the images of the different luminous 

 points of the ground on the retina and a similar arrangement of 

 the images of the luminous points on the skin of the fishes", and 

 concludes that "vision is a kind of telephotography". 



A careful examination of Sumner's excellent photographs of 

 patterns produced in the skin of flatfishes by different backgrounds 

 shows clearly that the spacial arrangement of light and dark areas 

 in the skin is similar in all. It is essentially the same in fishes 

 over a background consisting of alternate black and white squares 

 as it is over one consisting of alternate black and white stripes or 

 black spots on a white field or white spots on a black field or an 

 irregular arrangement of high light and shadows as is found in 

 nature on gravel bottoms. Sunnier says (p. 468): "Squares, cross- 

 bands, circles, etc.. were never copied in any true sense, by the 

 fishes." The size of the dark and light areas in the background 

 have a profound effect on the nature of the pattern, but I can find 

 no evidence indicating that their form or their spacial arrangment 

 has any. Where then is there any foundation for Loeb's specula- 

 tion on the mechanics of vision? What evidence is there that 

 images on the retina are reproduced as such in the brain? 



