ferent plants require different degrees of light and heat for the formation of their colors. Ferns and conifers need but little. Different colors require different degrees for their development; yellow is an exception, for it seems to be found equally well under the most varied circumstances. Some red flowers, as the tulips, color well in the dark, and the blues and violets seem partly independent partly dependent on the light. The crocus takes on its deep-blue violet in the dark, while the Prunella grandiflora remains white. A steady supply of nutriment is essential. Askenasy tried some cut branches of foxglove and other plants in a glass exposed to a strong light. The first flowers that came out were bright, but the others grew paler, and the last were nearly white. The failure of nourishment neutralized the stimulating effect of a strong light. The effect of these external agencies is dependent on the disposition of the flower to particular colors. They may aid the tendency, but do not perceptibly modify it. The supposition that particular seasons are favorable to the development of particular colors is not fully established; and the real amount and extent of the influence of changes of season on color is not ascertained. The investigation of the effect of the soil is attended with great difficulties. When we change the soil, we change other relations, as those of light and temperature, without being able to measure the part that each may have in producing the result we see. By introducing changes in all the conditions we cause an abrupt modification in the life of the plant, by which it becomes more docile to our treatment. Hence most of the variations take place under cultivation. Variation once started, further changes are comparatively easy. Natural selection operates to perpetuate variations. Those colors which stand out most distinct from the surrounding green attract the insects which act as fertilizing agents, and become predominant and permanent.
A Fortress of the Polished-Stone Age in Spain.—M. A. F. Nogués, mining engineer, gives in "La Nature" an account of a fortified camp of the polished-stone age, situated on the plateau of Maestrazgo, Spain, which was discovered by the Abbé Ambrosio Sans, and which he has recently visited. The work is situated on the elevation called the Muela de Chert, which constitutes the highest groups of hills of the plateau, rising 2,880 feet above the sea. The end of the plateau shows a steep limestone ledge inaccessible at almost every point. At a considerable distance from this point, the lines of the fort are marked by an irregularly disposed mass of weather browned stones running across the ridge which is the continuation of the Muela, and separating the higher parts of the eminence from the lower levels of the plateau. A real fortified inclosure is thus formed, defended on one side by the natural precipitous escarpment, and by the wall along the rest of the circuit. The curved part of the wall, for a length of about eight hundred feet, is built of stones without mortar, arranged from the level, not of the surface of the ground, but of excavations made in the mountain by the prehistoric builders. On the south, the wall ends at a short distance before reaching the precipice; and here are found traces of an opening or the gate to the inclosure, which is about eight feet wide at the bottom. In the interior of the fortification are a smaller wall, still intact, and piles of stones, the remains of former habitations, the greater part of which have, however, disappeared. Those which have been traced were of an oval form, about twenty feet long by six and a half feet wide, sometimes grouped, sometimes isolated, but arranged apparently so as to conform to some politic regulation. When the ruins were first discovered, the bones of many animals which are now extinct in Spain, as well as those of some still living in the country, were found at the bottom of the wall. Without the inclosure were found stone implements, polished hatchets of a white, reddish-veined quartz, lance-points of blackish dioritc, and other objects of the polished stone age. Such a work could only have been constructed by a settled population, who had already attained considerable numbers. No traditions of the historic period mention such fortifications; and on this fact, as well as on the occurrence of the remains of animals that have not lived in Spain for time immemorial, is based the presumption that the structure dates from
