definite number of teeth of the reptilian jaw, is a natural preliminary condition to the high specialization of the teeth, with particular form for each.

The selection and specialization seem to be brought about by the suppression of part of the multiple series, and the modification of the teeth retained in different parts of the jaw for special function.

In the primitive Marsupials and Insectivores, he observes, the regular reptilian succession was early interrupted, while in all the higher mammals the reptilian succession of two series was retained in the anterior part of the jaw. In the Edentates and whales retrogression takes place in fins as well as in teeth; it is the first set of teeth that persists, the second set being represented by a rudimental row of tooth-caps buried in the jaw.* He concludes that there is strong evidence that the stem mammals had a uniform number of each kind of teeth and a uniform dental formula; that homodontism is secondary, and was actually preceded in time by heterodontism in the mammalian dentition.

The ancestral formula for both Marsupials and Placentals, according to this author, is: incisors 4, canines and premolars 5, molars 4. By adopting Röse's suggestion that incisor 5 of the marsupials belongs with the second series of incisors, he supposes that Placentals have lost one incisor and one molar from the primitive formula. The paper is an important contribution to the interpretation of the method of evolution, and must be studied with care to be fully appreciated; the author's conclusions are quoted on page 324.

For the purposes of this treatise a sufficient number of illustrative cases has now been presented to show where the emphasis is placed by the facts of geological biology as to the true factors of evolution. A great many examples crowd themselves upon the attention which must be left for the student to investigate directly and in detail. The evidence to be derived from the study of living plants and animals is so vast, that a special treatise would be necessary to do justice

to either, and the reader may find many admirable treatises giving account of this aspect of evolution.

Method and Purpose in the Selection of the Evidence here Set Forth. The facts which have been selected in these chapters have been chosen for the purpose of ascertaining what the geological history of organisms has been.

Examples have been taken and analyzed to ascertain what has been the particular law of succession in particular cases where the evidence was full enough to be relied upon. If the interpretation of these selected cases has been correct, the principles discovered may be applied to other cases.

The facts have been examined for the purpose of learning (1) what the fossils indicate has been the order of succession in the initiation of different forms of organisms; (2) what relation this succession bears to the relative importance of the characters in the economy of the individual organism, as shown by the systematic classification of the Animal Kingdom; and (3) what have been the determining causes by which the multitudinous differences in organic structure have been brought about. The first consideration in their selection was that they should be from among those of which the most perfect record is preserved. The cases already cited in evidence are not selected because they are the most important examples, nor because they illustrate only the most important laws of evolution, but they are selected because they are the best examples to show what the geological records testify regarding the history of organisms.

Different Kinds of Evidence Borne by Living and Fossil Organisms. Living organisms present the best evidence of the laws of ontogenetic development, because they furnish illustration of each stage in the development. A continuous series of the stages of development of a single organism is more satisfactory evidence of the essential nature of that development, than would be any number of detached exhibitions of sundry stages of development of different organisms.

So it is believed that the evidence borne by a series of fossils preserved in each stage of the geological record, of which specimens are well preserved and described from the first to the last, and which show the beginning, dominance,

decrease, and extinction of the type they represent, is of the highest value as evidence of the actual order of evolution. and of the general laws by which differentiation of form has taken place. And a few such cases far outweigh any number of detached specimens tied together by theoretical links.

Natural Selection seems Reasonable when Based alone upon the Study of Living Organisms. When we observe living animals in competition-the vigorous ones living and the weaker dying, the strong overcoming and devouring the weak, the large and fewer in number making their daily food of the smaller and more abundantly produced; when we note how the places for the greatest abundance of individuals are determined by the presence of favorable conditions for obtaining food; and thus, in general, when we observe that animals as they are are actually adjusted, each to its own most favorable conditions of environment-it seems reasonable to draw the conclusion that the differences distinguishing one animal from another may have arisen as the result of better fitness for the struggle for existence on the part of those which survived and carried on the race.

Having once assumed that the law of evolution is a process in which the chief active determining force has been natural selection by the survival of the fittest, it was easy to find illustrations of adjustment of structure and function to the conditions of environment among fossil, as has been done. among living, organisms.

Every Species of Organism that has Flourished in the Past the Fittest for its Place and Generation.-When, however, we stop one moment to consider the relations of organisms in the past to their own environment, it becomes evident that, at every particular stage in the geological history of organisms, the individuals then existing must have been as thoroughly well adapted to live under the conditions of their environment as the present inhabitants are adapted to live in their environEvery organism that has lived on the earth has in some sense been the fittest to live in the particular time and conditions it occupied.

ment.

If environmental conditions (outside of organic environment) have determined the evolution of organisms, then we

are obliged to assume a degree and amount of change in them. of which the facts of geology give no evidence.

If the conditions which have changed with the geological ages have been the organisms themselves, and they have constituted the environment, then it becomes necessary to explain the more powerful contestants before their selecting agency can result in the survival of fitter races.

But leaving aside for the present the philosophical argument, the burden of these pages is to show what is in fact the testimony on these questions furnished by the organic history as found in the best-preserved parts of the record.

As previously explained, the records which are made at the place and time of the formation of the rocks are those which must on that account be the most perfect we can consult. The rocks bearing fossils are not wholly, but are in the large majority of cases, of marine origin. This determined the selection of the evidence from among marine animals. The animals of which the best records could be preserved in the rocks are those secreting hard parts-shells, or corals, or similar parts; hence the examples have been taken chiefly from the corals, the Mollusca, and Brachiopods.

The Geological Evidence does not Emphasize the Importance of Natural Selection as a Factor of Evolution.-What has already been said is sufficient to show that the emphasis of the testimony brought forward differs from the emphasis drawn by the embryologist, or by the student of living organisms, as to the relative prominence of the several factors in the evolutional history of organisms.

That which has seemed most conspicuous to the latter class of observers has been the intimate relationship existing between morphological difference and environmental conditions; paleontological facts point to the greater importance of the continuous and progressive process of differentiation and specialization of structure and function with the passage of geological time.

The law of natural selection, suggested to explain the evolution from the first point of view, calls for an extremely slow rate of modification, but uniform and continuous. The facts of the history itself point to the reality of rapid strides at

critical points, with long periods of almost absolute cessation of progress; and suggest that the part played by what is called natural selection has determined rather the particular individuals and the place and time for advance steps, than, either the direction of the steps themselves, or the relative value of the particular modifications in relation to continuation of the race, which have taken place.

The study of the actual facts of the geological history of organisms points unmistakably to a course of evolution by descent, in which the progress attained by each succeeding form was a paramount condition of the origin of the next member of the race.

Objection may be taken to an argument based on so few examples. I think the force of this objection will be lessened when we bear in mind that the examples were selected primarily because of their fitness to testify upon the points in question, viz., the law of the history of organisms, the nature, the rate, and the order of modification of form, which organisms actually undergo in producing that divergence of specific forms observed at any particular stage of the history.

It may be said that the particular kinds of animals selected do not fairly represent the total life of the world. To this objection the reply may be made that a full quota of diversity of specific forms has been attained by the races examined, and the chief question before us is, How has that diversity arisen?

If the facts we have examined do not support the hypothesis that the chief factor in organic evolution is either external environment or natural selection, it is not on account of any lack of fitness to testify on this point, if it were true.

The facts examined-and we believe that fuller examination of other statistics, both fossil and recent, will support the same conclusion-show that evolution is rather an intrinsic. law of all organisms, and is to be discovered in the phenomena of variation, which appear to be constantly active, rather than in any accidental operations dependent upon the conditions of external environment.

The emphasis is placed upon the intrinsic rather than the extrinsic factors of evolution, as the actual determinants of