descent; Glyphioceras goes through distinctly the stages from Anarcestes of the Devonian, through Tornoceras and Prionoceras, finally stopping in the Glyphioceras stage. Schloenbachia hastens through the Anarcestes, Tornoceras, and Prionoceras so that they are hardly recognisable, and gets to be a Glyphioceras even in its middle larval stage, goes through two more goniatite stages, and several ammonite stages before it becomes a Schloenbachia. DEVELOPMENT OF GLYPHIOCERAS. Glyphioceras in its development does not show the Bactrites (Plate V, Fig. 7) and Mimoceras (Plate V, Fig. 8) stages, so these must be studied in lower forms; but from the Anarcestes stage up they are sharply defined. It begins life as a protoconch or embryonic shell while undoubtedly still in the egg; this (shown on Plate I, Figs. 1-5) corresponds to the primitive cephalopod. At the beginning of its larval stage the animal left the protoconch, built up the first body chamber, and cut off the embryonic part of the shell by the first septum (Plate I, Figs. 6 and 10, Plate IV, Fig. 1, 1); at this stage the shell is analogous to the primitive nautiloid, and it is called in Hyatt's nomenclature ananepionic. With the second chamber the young shell becomes an ammonoid, and corresponds to the genus Anarcestes of the Lower Devonian; this is shown on Plate I, Fig. 6, and on the second and third septum of Fig. 9, also Plate IV, Fig. 1, second and third sutures. On Plate IV, Fig. 7 shows for comparison the septa of Anarcestes subnautilinus Sandberger. At the fourth suture the shell is transitional to Tornoceras of the Middle Devonian. Plate I, Fig. 9, shows the beginning of the Tornoceras stage, which lasts through the sixth chamber, as shown on Fig. 12, and on Plate IV, Fig. 1, at the fourth and fifth septa. For comparison the development of the septa of Tornoceras retrorsum Buch is figured on Plate IV, Fig. 8. At the seventh chamber, three fourths of a whorl, and diameter of about 0.85 millimetre, the shell changes its form rather suddenly, the umbilicus widens, the body chamber narrows, and the number of lobes and saddles increases; this stage corresponds to the Upper Devonian genus Prionoceras, as shown on Plate I, Figs. 11 and 12; and more advanced on Plate II, Figs. 2-6, the resemblance to that genus being perfect in everything except size, and any naturalist would have described these stages as Prionoceras if they had not been taken out of the inner coils of Glyphioceras. The septa of this stage are shown on Plate IV, Fig. 1, eighth septum, and Figs. 2, 3, and 4; Fig. 5 shows the transition to Glyphioceras, and the corresponding shell is shown on Plate II, Figs. 7 and 8. The adult is shown on Plate II, Fig. 9, and Plate IV, Fig. 6. DEVELOPMENT OF SCHLOENBACHIA. Schloenbachia begins its development almost where Glyphioceras leaves off, or rather it hastens through the stages before the Glyphioceras stage so rapidly that they are almost unrecognisable. On Plate III, Fig. 3 shows the embryonic protoconch, and the first six septa drawn as if unrolled, in which short space it hastens through the stages corresponding to Anarcestes, Tornoceras, Prionoceras, and becomes a Glyphioceras. Fig. 4 shows the shell in that stage, and Fig. 6 shows the corresponding septa. Fig. 7 shows the transition to the Gastrioceras stage, the septa of which are seen in Fig. 8. The next stage corresponds to the Carboniferous genus Paralegoceras, Figs. 9 and 10, and with this the goniatitic larval period ends. The first adolescent (ammonitic) character that appears is a keel, at the diameter of 2.7 millimetres, and shortly after this the first lateral saddle becomes divided by a secondary lobe (Plate III, Fig. 11), and the whorl becomes higher and the spiral wider (Fig. 12). Shortly after this the lobes and saddles all become slightly digitate (Fig. 13), and the family relationship of the young shell are unmistakable. Fig. 1 shows a cross-section of an adolescent shell, four whorls, in which the broad, low helmet-shaped inner whorls, the widening of the umbilicus, increase in height of the later whorls, development of the keel, and flattening of the sides are shown, seven and a half times enlarged. Fig. 2 shows an adult cross-section, six whorls, one and three quarter times enlarged, showing the angular shoulders and considerable involution of the adult shell. Since Schloenbachia appears near the time of final extinction of the ammonites, and is still normal in development, it gives in its own development an admirable epitome of the history of the race. And by combining this with the ontogeny of its ancestor, Glyphioceras, we are able to trace the genealogy with certainty back to the first ammonoids that appeared in geologic history. By following this method the complete ontogeny of any species of ammonite may be worked out, and in order to learn the phylogeny of any form it is only necessary to combine this with comparative study of antecedent genera and species. When this is done for all the Ammonoidea, their genealogy will be more perfectly known than any other family tree possibly can be. If evolution needs any demonstration to raise it from a working hypothesis to a fixed principle of biology we have it in the history of the fossil cephalopods. X. THE EVOLUTION OF THE MIND. "Three roots bear up dominion, Knowledge, Will, Mind the sum total of psychic changes. LOWELL. THE mind, in the sense in which I shall use the word here, is the collective function of the sensorium or brain of man and animals. It is the sum total of all psychic changes, actions, and reactions. Under the head of psychic functions are included all operations of the nervous system, as well as operations of likę nature which take place in creatures without specialized nerve fibres or nerve cells. As thus defined, mental operations are not necessarily or exclusively conscious. With the lower animals nearly all of them are automatic and unconscious. Even with man, most of them must be so. But between the automatic Mind not consciousness. and the conscious actions no sharp line of division exists. Consciousness is not an entity but a condition. It stands related to mind much as flame is related to fire. All functions of the nervous system are alike in essential nature, and from the present point of view may be considered together. It is a recognised law in biology that "function precedes structure." To define this law more exactly we should say that function precedes the differentiation of the organ on which it depends. There is a certain work to be done and a certain body of cells are set apart Function precedes structure. sooner or later to do it. Just as ploughing was done in some fashion before the invention of the plough, so in some manner respiration was accomplished before the development of gills and lungs. Something of mental action came before there was an organized brain. This law involves nothing mysterious or incomprehensible. It does not, so far as we know, imply the preexistence of mind or the carrying out of any predetermined purpose in development. All this may be or may not be, but the phenomena in question throw no light on it. The fact seems to be that when the bodily processes make certain demands on an organism, these demands will be met in some fashion. Through natural selection some better structure will come into competition. The cells and tissues on which the function depends will be specialized as an organ. In creatures of different ancestry the same function may be discharged by widely different organs. Conversely, what is ancestrally the same organ may in different groups of animals serve functions widely different. In the animals of one cell, or protozoa, breathing and digestion are each performed by the whole body. In the division of labour or specialization which arises in the higher or many-celled animals certain alliances of cells or tissues are set apart for respiration alone, and certain others for digestion, while other functions of animal life are relegated to still other cell alliances. Each organ in turn is released from all functions except its own. Irritability, or the response to external stimulus, is an attribute of all living organisms. In the method |