the original formation of these organs in their ancestral forms, their development in the individual, the phylogenetic evolution of function, and the great variation resulting from individual adaptation. Before entering upon our immediate subject, it would be as well to define its limits by saying a few words concerning the questions not under immediate consideration. 1. Original formation of organs in ancestral forms. Little is known concerning the primary origin of organs, and their development before their assumption of the aspect and function by which we know them. What were the rudimentary leaves like in the ancestors of flowering plants? What were the eyes of the first vertebrates like? Did these organs develop from existing organs fulfilling other functions, or were they formed independently? However that may be, if they arose from other organs, we know nothing of the modifications which they underwent; and if they were formed independently, we need not discuss the fact here, for in that case the organ would not have developed by transformation. Having once been formed, it would develop and improve, and this process would not necessarily have been attended by partial degeneration. 2. Development of organs in the individual.-In the course of embryological development, organs do not exactly repeat the successive phases A B C D through which they passed during their ancestral evolution. Generally speaking, and especially in the case of plants, the development of organs in an individual is direct, and gives no clue to its ancestral history. Moreover, when there is a recapitulation of ancestral stages, it often happens that evolution takes place without leaving traces of the various stages. This is especially the case in complex organs which have been produced by many lines of evolution converging in a single struc ture a struc edc ba ture which thus becomes the seat FIG. 1.—Diagram showing the evolution of pyramidal of a special func cells in the animal series. The upper series of cells represents the psychic cell in tion or set of various vertebrates: A, the frog; B, the lizard; C, the rat; D, man. The lower series shows the functions. The neuron, nerve process and of the terminal ramifications; for instance, the c, the nerve more fully developed; d, appearance of lateral branches of the axis cylinder; e, development ganglionic of protoplasmic outgrowths of the protoplasm of nerve-cell and nerve. (Ramon y Cajal.) cell of the cortex of the human brain, passes successively through stages corresponding to those which are to be found in the adult fish, frog, bird, and mammal. In this case the development consists in an increasing complexity of the cell with no formation of unnecessary rudimentary parts. 3. Phylogenetic evolution of function.-Evolution may be regarded from a physiological, as well as from an anatomical standpoint,1 but, in the former case, evolution is less a set of changes of function than an increasing specialization and division of labour, and under these circumstances it is often difficult to recognize a degenerative element in the evolution. A few examples will demonstrate this point: Self-mutilation is a very common phenomenon among Echinoderms. Among brittle star-fish this reaction is controlled by some region of the nervous system; in some star-fish the reaction follows more quickly because the stimulus can act upon the ganglion at the root of each arm near the circumoral nerve-collar. In Asteracanthum rubens, there is a complete localization of this function, and selfmutilization only results when an exact region of the nervous system is stimulated.2 In the Medusa we find an equally interesting example of functional evolution (Romanes). With some of these (the 1 Evolution fonctionnelle du système nerveux. J. Demoor, Revue universitaire, Bruxelles, 1892. 2 Contribution à la physiologie nerveuse des Echinodermes. J. Demoor and M. Chapeaux. Tijds. Ned. Dierk. Vereen. (2) III. 2 Nov. 1891. 3 Preliminary Observations on the Locomotor System of Medusa ; Jelly-fish, Star-fish, and Sea-urchins. Romanes, Int. Scient. Series, Acraspedote, on the outside of the umbrella being separated from the central part, the two separate parts continue to lash the water, the outer part with even strokes, the central mass more slowly and feebly. With the Craspedote, on the other hand, the central part, under the same conditions seems quite paralyzed and immovable, while the outer part continues to move in a perfectly normal The causative function of the movement, the spontaneity of the movement as it was formerly called in physiology, is incompletely specialized in the Acraspedote, whereas in the Craspedote it is entirely localized. manner. Individual adaptation. The individual is by no means a slave to heredity. It is capable of. certain modifications under the influence of certain external conditions. These phenomena of individual adaptation may be arranged in three groups. (a) When an organism, either animal or vegetable, is placed under new conditions of existence, when for instance, it relinquishes a terrestrial for an aquatic life, light for darkness, or fresh water for salt or esturine water, its external aspect, and internal structure, undergo variations of considerable importance if it succeeds in adapting itself to the new conditions.1 1 EXAMPLES: (a) The leaves of the water Ranunculus with lacinated leaves (Ranunculus aquatilis fluitans, etc.), are of normal structure when cultivated on dry land. The epidermis is furnished with stomata and the constituent cells contain no chlorophyll. The organ does not, however, lose its primitive and typical characters. Actual organic transformation cannot, therefore, be said to take place in the case of individual adaptation. The same leaves of the same plant when grown in water are much longer than those of the terrestrial type; the leaves have no stomata, and the epidermic cells are full of chlorophyll (Askenasy, Ueber den Einfluss des Wachstumsmediums auf die Gestalt der Pflanzen, Bot. Zeit., 1870, pp. 193 and following). Among the Stratiotes aloides it is not uncommon for the upper part of a leaf to rise above the surface of the water while the base is submerged; the epidermis at the base contains chlorophyll, but has no stomata, while the part of the same leaf which rises out of the water is furnished with stomata, but has no chlorophyll in the epidermis. (b) A good example of this individual adaptation may be obtained by cultivating Cacti alternately in the light and in the dark. Goebel has shown that when a specimen of Phyllocactus is cultivated in the dark, the stems are prismatic and thorny; if the plant is afterwards placed in the light, the thorns disappear and the stems become quite smooth. (K. Goebel, Ueber die Einwirkung des Lichtes auf die Gestaltung der Kacteen und anderer Pflanzen, Flora, vol. 80, p. 96, 1895.) (c) The animal kingdom furnishes numerous examples of individual adaptation. The gradual drying up of Lake Aral caused the formation of a number of basins containing water at various stages of concentration. The Cardium of this region exhibits a whole series of adaptive variations. The shells become thinner and horny, their shape elongates, the openings contract, and their colour becomes duller (Bateson). Mytilus edulis (the edible mussel), exhibits three different kinds of shells. It lives either in salt water, deep water, or shallow water visited by the tide. In each of these three vicinities the shells exhibit typical aspects. The direction of boney lamellæ is known to agree with that in which the greatest strain is habitually applied, and the entire structure of a bone is dominated by the incidences of the forces applied to it. When, after a badly-mended fracture, the two broken |