IMMEDIATE CAUSATION 151 



Immediate Causation — It is evident from the table that there are few 

 important vertebrate types about whose photomechanical changes, or 

 lack of them, we cannot make positive statements in a descriptive way; 

 and the reader has just been promised an integration of the apparent 

 hodge-podge of their distribution, and an interpretation of the phylo- 

 genetic degeneration of these ingenious phenomena. But there is no 

 branch of physiology which is in a less satisfactory state than the whole 

 matter of the immediate nature, causation, and control of the photo- 

 mechanical changes. 



The unwieldy literature of the subject is full of contradictory con- 

 clusions based on seemingly equally sound lines of evidence, on failures 

 to take account of the great individual physiological variability of the 

 lower vertebrates (particularly the amphibians!), and on simple ignor- 

 ance of other workers' results, especially of conclusive negations of some 

 of the pioneering researches. 



We do not even know whether either extreme position of the retinal 

 pigment or of the rod or cone myoids represents a condition of relax- 

 ation, or whether the expansion and retraction of the pigment and the 

 elongation and shortening of myoids are all active processes. We can 

 glibly say that the movements of the pigment are "due to protoplasmic 

 streaming" but (though it could easily be ascertained on isolated epi- 

 thelium in vitro) we do not know whether the pigment cell can respond 

 in either direction, autonomously and directly, to light and darkness. 

 We have no conception whatever of the intracellular mechanism which 

 changes the length of a myoid, though these changes are enormously 

 greater than those which take place in a striated muscle cell whose 

 fibrillar machinery is fully revealed to microscopic view. And we are 

 greatly puzzled by the fact that apparently the same stimulus, or lack 

 of stimulus, which causes the myoid of one visual-cell type to elongate, 

 simultaneously causes the other to shorten. It is known that myoid vol- 

 umes remain constant at all lengths, and that shape-changes elsewhere 

 in the visual cells are purely passive. It is known that contractile cone 

 myoids respond by shortening in the presence of acids, and that retinae 

 which contain cones are acid in the light and alkaline in the dark. 

 Studnitz has injected phosphoric acid into dark-adapted fishes and found 

 that the pigment and cones took their 'light' positions. Injections of 

 alkali into light-adapted fishes made them dark-adapt. Studnitz thinks 

 that even the effects of adrenalin chloride are due to its acidity, not to 

 the hormonal base. To what it is that rod myoids respond, and how, no 



