PHOSPHOEESCENT ORGANS. 85 



'Porcupine' expedition (1869-70). This hypothesis, which is 

 known as the Theory of Abyssal Light, consists essentially in the 

 idea that the light diffused by phosphorescent creatures is 

 capable of taking the place of sunlight in those depths which 

 the rays of the sun cannot penetrate. It is evident that the 

 correctness of this idea cannot possibly be experimentally tested 

 and proved, but at the same time we cannot but admit that it 

 is highly probable. For although it has been argued, as an 

 objection to this idea, that phosphorescence is not an exclusive 

 peculiarity of deep-sea creatiu-es, but on the contrary, so far as 

 we know, occurs more frequently among animals living on the 

 surface, this objection must certainly be considered as anything 

 but conclusive. We know from exact experiment in individual 

 cases, particularly on glowworms, that phosphorescence is the 

 product of a chemico-physiological process in the living body of 

 the animal, exactly as carbonic acid is a natural product of 

 respiration. What requires us to assume that this ought to 

 occur in deep-sea animals only,^^ supposing that theory to be 

 accurate ? The obvious ground of this objection is the tacit 

 assumption that, if phosphoric light can really be of use to 

 any creature, it must only occur in cases where it covdd be 

 utUised. But this mode of argument offers an example of a 

 very common but very gross eiTor : the idea, namely, that the 

 effect produced by the function of an organ, or that the function 

 itself — in the present instance the production of light in the 

 light- organ — can be brought into existence by reason of the 

 usefulness of its results, when the use, in fact, makes its 

 appearance at a later period. Phosphorescence, as it is 

 developed in the living tissue of animals living at the surface, 

 may perhaps never be of any use to the creatures that possess it, 

 nor to the enemies that pursue them. But the same effect of a 

 similar chemical process may nevertheless be advantageous to 

 other creatures, which, like the deep-sea animals, would other- 

 wise be condemned without exception to live in total darkness. 

 We are not at present acquainted either with the various che- 

 mical processes by which phosphoric light is produced in dif- 

 ferent animals, nor with the uses which these processes may 

 subserve for the animals themselves ; but we know of some 



