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Of much greater significance than this observation are the facts which show that a large number of animals assume； to some extent； the colour of the ground on which they are placed。 Pouchet found through experiments upon crustaceans and fish that this influence of the ground on the colour of animals is produced through the medium of the eyes。 If the eyes are removed or the animals made blind in another way these phenomena cease。 The second general fact found by Pouchet was that the variation in the colour of the animal is brought about through an action of the nerves on the pigment…cells of the skin； the nerve…action being induced through the agency of the eye。
The mechanism and the conditions for the change in colouration were made clear through the beautiful investigations of Keeble and Gamble； on the colour…change in crustaceans。 According to these authors the pigment…cells can； as a rule； be considered as consisting of a central body from which a system of more or less complicated ramifications or processes spreads out in all directions。 As a rule； the centre of the cell contains one or more different pigments which under the influence of nerves can spread out separately or together into the ramifications。 These phenomena of spreading and retraction of the pigments into or from the ramifications of the pigment…cells form on the whole the basis for the colour changes under the influence of environment。 Thus Keeble and Gamble observed that Macromysis flexuosa appears transparent and colourless or grey on sandy ground。 On a dark ground their colour becomes darker。 These animals have two pigments in their chromatophores； a brown pigment and a whitish or yellow pigment； the former is much more plentiful than the latter。 When the animal appears transparent all the pigment is contained in the centre of the cells； while the ramifications are free from pigment。 When the animal appears brown both pigments are spread out into the ramifications。 In the condition of maximal spreading the animals appear black。
This is a comparatively simple case。 Much more complicated conditions were found by Keeble and Gamble in other crustaceans； e。g。 in Hippolyte cranchii； but the influence of the surroundings upon the colouration of this form was also satisfactorily analysed by these authors。
While many animals show transitory changes in colour under the influence of their surroundings； in a few cases permanent changes can be produced。 The best examples of this are those which were observed by Poulton in the chrysalids of various butterflies； especially the small tortoise…shell。 These experiments are so well known that a short reference to them will suffice。 Poulton （Poulton； E。B。； 〃Colours of Animals〃 （The International Scientific Series）； London； 1890； page 121。） found that in gilt or white surroundings the pupae became light coloured and there was often an immense development of the golden spots； 〃so that in many cases the whole surface of the pupae glittered with an apparent metallic lustre。 So remarkable was the appearance that a physicist to whom I showed the chrysalids； suggested that I had played a trick and had covered them with goldleaf。〃 When black surroundings were used 〃the pupae were as a rule extremely dark； with only the smallest trace； and often no trace at all； of the golden spots which are so conspicuous in the lighter form。〃 The susceptibility of the animal to this influence of its surroundings was found to be greatest during a definite period when the caterpillar undergoes the metamorphosis into the chrysalis stage。 As far as the writer is aware； no physico…chemical explanation； except possibly Wiener's suggestion of colour…photography by mechanical colour adaptation； has ever been offered for the results of the type of those observed by Poulton。
V。 EFFECTS OF GRAVITATION。
（a） EXPERIMENTS ON THE EGG OF THE FROG。
Gravitation can only indirectly affect life…phenomena； namely； when we have in a cell two different non…miscible liquids （or a liquid and a solid） of different specific gravity； so that a change in the position of the cell or the organ may give results which can be traced to a change in the position of the two substances。 This is very nicely illustrated by the frog's egg； which has two layers of very viscous protoplasm one of which is black and one white。 The dark one occupies normally the upper position in the egg and may therefore be assumed to possess a smaller specific gravity than the white substance。 When the egg is turned with the white pole upwards a tendency of the white protoplasm to flow down again manifests itself。 It is； however； possible to prevent or retard this rotation of the highly viscous protoplasm； by compressing the eggs between horizontal glass plates。 Such compression experiments may lead to rather interesting results； as O。 Schultze first pointed out。 Pflueger had already shown that the first plane of division in a fertilised frog's egg is vertical and Roux established the fact that the first plane of division is identical with the plane of symmetry of the later embryo。 Schultze found that if the frog's egg is turned upside down at the time of its first division and kept in this abnormal position； through compression between two glass plates for about 20 hours； a small number of eggs may give rise to twins。 It is possible； in this case； that the tendency of the black part of the egg to rotate upwards along the surface of the egg leads to a separation of its first cells； such a separation leading to the formation of twins。
T。H。 Morgan made an interesting additional observation。 He destroyed one half of the egg after the first segmentation and found that the half which remained alive gave rise to only one half of an embryo； thus confirming an older observation of Roux。 When； however； Morgan put the egg upside down after the destruction of one of the first two cells； and compressed the eggs between two glass plates； the surviving half of the egg gave rise to a perfect embryo of half size （and not to a half embryo of normal size as before。） Obviously in this case the tendency of the protoplasm to flow back to its normal position was partially successful and led to a partial or complete separation of the living from the dead half； whereby the former was enabled to form a whole embryo； which； of course； possessed only half the size of an embryo originating from a whole egg。
（b） EXPERIMENTS ON HYDROIDS。
A striking influence of gravitation can be observed in a hydroid； Antennularia antennina； from the bay of Naples。 This hydroid consists of a long straight main stem which grows vertically upwards and which has at regular intervals very fine and short bristle…like lateral branches； on the upper side of which the polyps grow。 The main stem is negatively geotropic； i。e。 its apex continues to grow vertically upwards when we put it obliquely into the aquarium； while the roots grow vertically downwards。 The writer observed that when the stem is put horizontally into the water the short lateral branches on the lower side give rise to an altogether different kind of organ； namely； to roots； and these roots grow indefinitely in length and attach themsel