174 NATURAL SCIENCE. march. 



secondarily acquired. But the fact that the nearly related slugs show 

 no such sensitiveness to light seems to indicate that the second 

 hypothesis is more likely to be correct. On the other hand, in the 

 case of blinded newts, which select the darker places in which to lurk, 

 as if their skins were in some way sensitive to light, this might be 

 thought more likely to be the primitive function retained, otherwise it 

 is not easy to see why it should have been required in addition to 

 eyes. The reported sensitiveness to light of the blind cave-dweller, 

 Proteus anguineus, is a quite intelligible adaptation to the needs of the 

 animal. 



Here, then, we are brought face to face with a very fascinating 

 problem. The skins of animals may possess a sensitiveness to light 

 which must in some way be correlated with the development of eyes. 

 Have we any clue which will explain the facts, and also the connec- 

 tion between the diffuse light-sensation and the specialised light- 

 sensation of our eyes ? All who are interested in this — and who is 

 not ? — will naturally welcome any book dealing specially with this 

 subject. Probably all that can be said about it from the generally 

 accepted point of view can be found in Dr. Willibald Nagel's 120 

 pages, published by Gustav Fischer, of Jena, under the title of " Der 

 Lichtsinn augenloser Tiere." Excellent as Dr. Nagel's treatment of 

 the subject is, there is, as we shall see later on, a factor in the 

 problem which Dr. Nagel is not alone in too lightly dismissing, 

 though due attention to it would have brought him nearer to some 

 positive and satisfactory conclusion. 



The first part of the work is a reprint of a lecture under the 

 somewhat startling title of " Seeing without Eyes " (Sehen ohne 

 Augen). The second part contains a description of experiments 

 made by the author in the Naples Aquarium and elsewhere on the 

 reactions, chiefly of molluscs, to sudden positive or negative variations 

 of light-intensity. The third part consists of five short appendices or 

 notes, expanding certain subjects which had been too briefly dealt 

 with in the lecture ; and the volume concludes with a useful biblio- 

 graphy of treatises on the physiology of sensation in general, and of 

 sight in particular. 



In addition to the better known examples of the dermatoptic 

 function above mentioned, all of which are cited in Dr. Nagel's book, 

 we have the phenomena especially investigated and described by the 

 author himself. These require a brief notice. 



First of all, then, the author established beyond doubt that, 

 apart from the stimulus of very bright lights, it is the sudden changes 

 in the light-intensity to which skins endowed with the dermatoptic 

 function react, and further, that some animals react to sudden increase 

 of light, and others to sudden diminution. That the oyster, for 

 instance, should react to a sudden diminution of light had been 

 denied by Rawitz, more or less on theoretical grounds. The denial 

 and the objections are conclusively met by the author, the former 



