most nearly to a natural one because it is based on characters having physiological significance. It is based primarily on the structure of the endochrome, and secondarily on the method of forming auxospores and the general shape of the frustules. Van Heurck does not employ this system in his Synopsis because of the large number of fossil specimens and those from deep-sea soundings to which it could not be applied. But this is not a valid objection, for all the genera are represented by modern species, and these are sufficient for a basis of classifications, and since the specific characters are based mainly on the structure of the valves, there will be no trouble with the fossil forms. The following synopsis of Petit's system includes the higher divisions only. I. Bacillariacea coccochromaticæ. With numerous endochrome granules. One mother A. Frustules concentrically constructed. Melosi B. Frustules bilateral, one or two mother cells forming two auxospores, as far as known asexually. Fragilarieæ, etc. II. Bacillariacea placochromaticæ. With one or two large endochrome plates. A. One endochrome plate lying against the convex valve; one mother cell forming one auxospore asexually. Cocconeidex. B. A single endochrome plate extending diagonally C. Two endochrome plates lying next the two valves. Although Petit's system is by no means perfect, it is at least a step in the right direction. He bases it upon characters that have some physiological significance, while the other systems are wholly or in greater part based on merely accidental characters. A clue to the genetic relationships of Diatoms, as of other plants, will be most certainly found in their method of reproduction. The shape of the frustules, or their markings, will serve for specific, or in some cases for generic characters, but they have no significance that will warrant their use in the erection of higher groups. Absolute shape and size will not serve as definite characters, for a single species between one auxospore stage and the next varies greatly in both these respects. Owing to the peculiar mode of cell division in which each new valve is formed inside the old one, each new frustule is smaller than the parent, hence the size gradually decreases until an auxospore is formed. Schumann'2, out of 470 species. found ten in which the length of the largest was five times that of the smallest; twenty-nine in which the largest were from three to four times as long as the smallest, and the rest showing less variation. The variation in form is even as great as the variation in size. This is probably due to the difference in the thickness of the girdle, i. e. the part of the valves that overlaps, in different parts of the frustule. Navicula iridis Ehr. is a good example of a variable species. Its different forms have been described as species by most writers. In the typical form the valves are elliptical with gracefully curved margins. The first variation from this type has apices cuneate, and a still further deviation shows them acuminate-cuneate; and from this it varies to rostrate or capitate; and a diminution in size goes step by step with this change in form. These forms are represented by Navicula iridis Ehr., N. amphigomphus Ehr., N. affinis Ehr., N. amphirhynchus Ehr., and N. producta W. Sm. If the overlapping portions of the valves are slightly thicker near the ends than elsewhere, this variation would be the necessary result, for each new valve formed inside an old one would be slightly constricted opposite this thickened place, at first changing the rounded ends to cuneate, and as the narrowing pro12 Pfitzer, 1. c., p. 441. ceeded still further, the cuneate form would become rostrate and a still further narrowing would give a capitate form. So form and size, although they have a certain significance, are not to be considered infallible characters. The geological records throw no light upon the relationship of the Bacillariaceæ, for when this family first appeared, we find the same genera, and largely the same species as in our modern ones. This is probably due to the fact that their ancestors lacked the siliceous covering, and hence were not preserved. Diatoms evolved the same as all other plants until they developed their shells, but these put a stop to their further evolution, at least they show no trace of evolution since their first appearance. So the question arises whether the Diatoms represent the ends of several closely related genetic lines the further development of which was stopped by their siliceous shells, or whether we may trace the development of one form from another. The former supposition is the more probable, for the form of the earliest fossil specimens is identical with that of modern specimens of the same species; and the same genera are found among fossil as among modern Diatoms. If one genus of Diatoms developed from another, we ought to find the more primitive forms in the earlier strata, for there is little. chance that their remains would not be preserved had they existed. But instead of this, Diatoms of all forms appear almost simultaneously. We may conclude then that the Bacillariaceæ represent the silicified ends of several closely allied genetic lines and that they have not changed in form since they acquired their siliceous covering. The structure of the valves it follows will tell us practically nothing of their relationship. There are five methods by which auxospores are formed13. In the first the protoplasm of one frustule simply escapes from the valves, grows to a certain size, and then invests itself with new valves. In the second, two auxospores, instead of one, are formed in the same way by the dividing of the protoplasm of a single plant. In the third, the protoplasm of two Diatoms unites to form an auxospore. In the fourth, the protoplasm of 13 Murray, 1. c. two Diatoms emerges from the valves, and placed by side, but without conjugation, forms each an auxospore. In the fifth, two Diatoms divide transversely and the two halves of each conjugate, each half with the corresponding half of the other and thus form two auxospores. Before any truly natural classification can be made the significance of these various modes of producing auxospores must be understood. Whether the sexual or the asexual method is the primitive one must be known, or whether the different methods are so many expedients to overcome the difficulties imposed upon these plants by their siliceous shells. At present our knowledge of the structure and physiology of Diatoms is not sufficient to enable us to construct a perfectly natural system of classification, and until something better is proposed, Petit's may well be adopted, for although it is not wholly natural, it is more so than any which has preceded it. A NEW FACTOR IN EVOLUTION. BY J. MARK BALDWIN. (Continued from page 451). III. Social Heredity.-There follows also another resource in the matter of development. In all the higher reaches of development we find certain co-operative or "social" processes which directly supplement or add to the individual's private adaptations. In the lower forms it is called gregariousnes, in man sociality, and in the lowest creatures (except plants) there are suggestions of a sort of imitative and responsive action between creatures of the same species and in the same habitat. In all these cases it is evident that other living creatures constitute part of the environment of each, and many neuro-genetic and psycho-genetic accommodations have reference to or involve these other creatures. It is here that the principle of imitation gets tremendous significance; intelligence and vol ition, also, later on; and in human affairs it becomes social co-operation. Now it is evident that when young creatures have these imitative, intelligent, or quasi-social tendencies to any extent, they are able to pick up for themselves, by imitation, instruction, experience generally, the functions which their parents and other creatures perform in their presence. This then is a form of ontogenetic adaptation; it keeps these creatures alive, and so produces determinate variations in the way explained above. It is, therefore, a special, and from its wide. range, an extremely important instance of the general principle of Organic Selection. But it has a farther value. It keeps alive a series of functions which either are not yet, or never do become, congenital at all. It is a means of extra-organic transmission from generation to generation. It is really a form of heredity because (1) it is a handing down of physical functions; while it is not physical heredity. It is entitled to be called heredity for the further reason (2) that it directly influences physical heredity in the way mentioned, i. e., it keeps alive variations, thus sets the direction of ontogenetic adaptation, thereby influences the direction of the available congenital variations of the next generation, and so determines phylogenetic development. I have accordingly called it "Social Heredity" (ref. 2, chap. xii; ref. 3). In "Social Heredity," therefore, we have a more or less conservative, progressive, ontogenic atmosphere of which we may make certain remarks as follows: (1) It secures adaptations of individuals all through the animal world. "Instead of limiting this influence to human life, we have to extend it to all the gregarious animals, to all the creatures that have any ability to imitate, and finally to all animals who have consciousness sufficient to enable them to make adaptations of their own; for such creatures will have children that can do the same, and it is unnecessary to say that the children must inherit what their fathers did by intelligence, when they can do the same things by intelligence" (ref. 6). (2) It tends to set the direction of phylogenetic progress by Organic Selection, Sexual Selection, etc., i. e., it tends not only |