The object of this work is to fill a gap in the treatment of the subject of animal movement depending on light, and to explain it by a consideration of the actual facts. After stating that the effect of light upon animal movement is purely mechanical, and that it is governed partly by the action of the light as the exciting cause, and partly by the structure of the sensitive organisation, Dr. Loeb proceeds, "I will now prove that the direction of the light rays determines quite generally the movements induced in animals by the light, no less than the direction of plant movement, and that the orientation not only of plants but of animals, depends upon the bodily form of the latter, in so far as the dorsiventral animals themselves move with the median plane in the direction of the light rays," etc. The more refrangible are the rays of light the more efficacious is its mechanical action upon animal and plant movement, which is affected also by the constant intensity of the light and its temperature. Thus it appears that the moth's flight into a flame must be considered as the same mechanical process as, for instance, the motions of sunflowers, the growth of the sprouting axis in buds, etc. Dr. Loeb's conclusion that the circumstances which govern the movements of animals towards the light are conformable to those which have been already recognised in relation to plant-movement, is supported by numerous facts, which appear to fully establish the accuracy of his observations and deductions. The diligent author who is at present engaged in scientific investigations at the _stazione zoologica_ in Naples, has in the mean time published a series of further observations on the same question, all of which, as was to be expected, corroborate the propositions set forth in the above mentioned little book. We have before us two reprints, one from the _Biologische Centralblatt_, Vol. X, Nos. 5 and 6, 1890, the other the _Archiv f. d. ges. Phys._, Vol. XLVII, with one plate and two wood-cuts, the former treating of the heliotropism of the nauplii of _Balanus perforatus_, whose periodical migrations are shown to depend upon the action of the light, the latter discussing the common features of heliotropism in animals and plants. UNTERSUCHUNGEN ZUR PHYSIOLOGISCHEN MORPHOLOGIE DER THIERE. 1. UEBER HETEROMORPHOSE. By _Dr. Jacques Loeb_. With 1 plate and 3 figures. Würzburg: George Hertz.