To this the present writer does not care to add. According to some writers, as Herbert Spencer, this inheritance is a prominent factor in evolution itself. According to August Weismann, it is simply a myth invented to explain phenomena the causes of which are unknown. Most of the arguments on both sides, thus far, have been theoretical only, based on no inductive evidence, and in science arguments of this sort are without value. Both suppositions rest, as Prof. Henry Fairfield Osborn has said, less "in fact than the logical improbabilities of other theories.". "Certainly," Professor Osborn goes on to say, "we shall not assist research with any evolution factor grounded upon logic rather than upon inductive demonstration. A retrograde chapter in the history of science would open if we should do so, and should accept, as established, laws which rest so largely upon negative reasoning. Darwin's survival of the fittest we may alone regard as absolutely demonstated as a real factor without committing ourselves as to the origin of fitness. The (next) step is to recognise that there may be an unknown factor or factors which will cause quite as great a surprise as Darwin's. The feeling that there is such first came to the writer in 1890, in considering the want of an explanation for the definite and apparently purposeful character of certain variations. Since then a similar feeling has been voiced by Romanes and others, and quite lately by Scott, but the most extreme expression of it has recently come from Dr. Driesch in the implication that there is a factor not unknown but unknowable! . . . We are far from finally testing or dismissing these old factors, but the reaction from speculation upon them is itself a silent admission that we must reach out for some unknown quantity. If such does exist there is little hope that we shall discover it except The unknown factors. by the most laborious research; and while we may predict that conclusive evidence of its existence will be found in morphology, it is safe to add that the fortunate discoverer will be a physiologist. "Chief among the unknown factors are the relations between the various stages of development and the environment." Professor Osborn concludes this discussion with the belief that "progressive inheritance is rather a process of substitution of certain characters and potentialities than the actual elimination implied by Weismann. "My last word is," he says, "that we are entering the threshold of the evolution problem instead of standing within the portals. The harder tasks lie before us, not behind us, and their solution will carry us well into the twentieth century." IV. THE FACTORS OF ORGANIC EVOLUTION FROM THE STANDPOINT OF EMBRYOLOGY. BY PROF. EDWIN GRANT CONKLIN. OUR knowledge of the mechanics of evolution must always depend in large part upon the study of individual development. More than any other science, embryology holds the keys to the method of evolution. Embryology shows the meth od of evolution. If on togeny (life history of the individual) is not a true recapitulation it is at least a true type of evolution, and the study of the causes of development will go far to determine the factors of phylogeny or race development. The causes and methods of evolution are intimately bound up with those general phenomena of life, such as assimilation, growth, differentiation, metabolism, inher itance, and variation; and the evolution problem can never be solved except through a study of these general phenomena of life itself. Our great need at present is not to know more of the course of evolution, but to discover, if possible, the causes of growth, differentiation, repetition, and variation. All these general phenomena are most beautifully illustrated in the development of individual organisms, and because they are fundamental to any theory of evolution I shall dwell upon them rather than upon the evidences for the Lamarckian or the Darwinian factors. Statement of propositions. I call attention very briefly to the following propositions: 1. Development, and consequently evolution, is the result of the interaction of extrinsic and intrinsic causes. 2. Intrinsic causes are dependent upon protoplasmic structure. 3. Inherited characters must be predetermined in the structure of the germinal protoplasm. 4. Germinal, as compared with somatic,* protoplasm is relatively stable and continuous, but not absolutely so, as maintained by Weismann; therefore, extrinsic causes may modify both germinal and somatic protoplasm. 5. It is extremely difficult to determine whether or not extrinsic factors have modified the structure of the germinal protoplasm. This is illustrated by some of the evidences advanced for the inherited effects of diminished nutrition, changes in environment, use and disuse. 6. Experiment alone can furnish the crucial. tests of these Lamarckian factors. Causes of development. 1. The causes of development in general are usually recognised as twofold-extrinsic and intrinsic. As examples of extrinsic causes may be mentioned gravity, surface tension, light, heat, moisture, and chemism in general; examples of intrinsic causes are the non-exosmosis of salts from living bodies in water, the pouring of a glandular secretion or the sap of plants into a cavity under high pressure, the active changes in shape and position on the part of cells, assimilation, growth, division, etc. There is not, however, a uniformly sharp and distinct line of demarcation between these two factors of develop * Somatic cells are those composing the tissues of the body as distinguished from germ cells-those destined to form the new organism. ment. Phenomena once supposed to be due entirely to intrinsic causes are now known to be the result of extrinsic ones, and it is practically certain that this will be found true of still other phenomena. But although it is not possible to draw any hard and fast line between these two classes of causes, one can, in general, recognise a very marked difference between them. Extrinsic causes may, in large part, supply the stimulus and the energy for development, and may more or less modify its course; the intrinsic causes are of a much more complex character than the extrinsic ones, they are inherent in the living matter and in large part predetermine the course of development. In one form or another the distinction between these two classes of causes is recognised by all naturalists. Professor His calls the intrinsic causes "the law of growth," the extrinsic ones the conditions under which that law operates. These designations correspond, at least in part, to Professor Cope's anagenesis and katagenesis, and to Roux's "simple and complex components" of developmental processes. While it is necessary to emphasize the differences between these two classes of causes, it is not intended thereby to dogmatically assert their total difference in kind. It may well be that these extrinsic and intrinsic causes are totally different in kind, but in our present state of ignorance it would be unjustifiable to affirm it. On the other hand, it would be just as unwarrantable to dogmatically affirm that there is no difference in kind between these two classes of causes, and that, therefore, all vital phenomena are only the manifestations of heat, light, electricity, attraction, repulsion, chemism, and the like. It may be that this is true, but there is as yet no sufficient evidence for it, and to attempt, as certain dynamical and mechanical hypotheses do, to refer all vital phenomena directly to such simple components as |