640 F. W. GAMBLE AND F. W. KEERLE. 



the previous experiments on the effect of continuous dark- 

 ness, where the only source of error is the brief exposure of 

 the animals to light during- the recording of their condition. 

 If the periodicity were more perfect, it would be possible to 

 eliminate this source of error by leaving the animals in con- 

 stant darkness for a long time, and then opening some flasks 

 at morning, others at midday, and so on. Unfortunately the 

 periodicity is far from perfect, the habit is but imperfectly 

 acquired. After prolonged dark-exposure the animals may 

 appear in the nocturnal phase at any hour of the day. Other 

 difficulties in the way of bringing this method to a successful 

 issue are the facts that the prawns eat one another with pro- 

 voking frequency, or die moulting only too often. 



But we appeal also to another class of facts, against the 

 hypothetical criticism that the constant-light experiments 

 may only show that the prawns are extremely susceptible 

 to small changes of light-intensity. In the first place, we 

 have no evidence that when the light-intensity is already 

 high this susceptibility is a fact. In the second place, we 

 have the evidence of the 'Might-induced noctui-nes" already 

 described which we must now discuss. Prawns exposed to 

 a bright light — the incandescent — reflected from a porce- 

 lain surface pass rapidly into a green colour phase, and 

 toward evening they become brilliantly nocturnal. Toward 

 morning they revert to their green colour, which is the 

 "diurnal colour" of prawns subject to this high light-in- 

 tensity. Viewed in the light of what we now know of the 

 periodicity of colour-change, it no longer appears a paradox 

 that change towards darkness or to increased light-intensity 

 should result in the same nocturnal coloration. The habit 

 is the dominant factor in the change. Prawns tend to 

 "nocturne" toward evening, and the tendency is sufficient 

 to overcome the antagonistic direction which the light 

 stimulus would and often may induce. Just as a cup of 

 black coffee at evening may be inefficacious in disturbing the 

 periodic function of sleep in men, so, too, the stimulant light 

 may fail to dominate the nocturnal tendency in the prawn. 



