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, inheritance, 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.

mic structure.

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... Intrinsic causes are dependent upon protoplas3. 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

we

those named above is practically to make impossible at present any explanation of vital phenomena. "If we would advance without interruption," says Roux,* * " must be content, for many years to come, with an analysis into complex components."

2. We need not now further concern ourselves with an explanation of extrinsic causes or simple components, since this subject properly belongs to chemistry and physics. If, however, we examine more closely some of the intrinsic causes or complex components, we will find that they are always associated with more or less complex structures; in fact, they are dependent upon

Intrinsic causes arise from nature of protoplasm.

structure.

The smallest and simplest mass of protoplasm that can manifest all the fundamental phenomena of life, such as assimilation, growth, division, and metabolism, is an entire cell, nucleus and cytoplasm, and probably centrosome. The cell is composed, as microscopic study plainly reveals, of many dissimilar but perfectly coadapted parts, each performing its specific function, and it may therefore properly be called an organism.. Some phenomena of cell life may be directly referred to the various visible constituents of the cell, but many of them are evidently connected with structures which we can not see, structures which may perhaps never be seen, and yet which must be vastly more complex than the most complex molecules known to chemistry, and yet much more simple than the microsomes, centrosomes, and chromosomes which are visible in the cell. With these ultra-microscopical particles many of the most fundamental phenomena of life are associatedviz., assimilation, growth, metabolism, and probably * Wilhelm Roux. Einleitung: Archiv für Entwickelungsmechanik der Organism.

differentiation, repetition, and variation. These functions are so co-ordinated that there can be no question that the ultra-microscopical structure is an organization, with part coadapted to part. The organization of the cell, therefore, does not stop with what the microscope reveals, but must be supposed to extend to the smallest ultimate particles of living matter which manifest specific functions. These are the vital units so generally postulated, the "smallest parts" of living matter, as they were called by Brücke, who first demonstrated that they must exist; the "physiological units" of Spencer, the "gemmules" of Darwin, the "micella groups" of Nägeli, the "pangenes" of De Vries, the "plasomes" of Wiesner, the "idioblasts" of Hertwig, the "biophores" of Weismann. Such ultimate units have been found absolutely necessary to explain those most fundamental of all vital phenomena, assimilation and growth, while many other phenomena, especially particulate inheritance, the independent variability of parts, and the hereditary transmission of latent and patent characters, can at present only be explained by referring them to ultra-microscopical units of structure. To deny that there are such units does not simplify the problem, as some seem to suppose, but renders it impossible of approach. A corpuscular hypothesis of life, like that of light, may be only a temporary makeshift, but it is better than nothing.

Whitman* well says: "Brücke's great merit consists in this, that he taught us the necessity of assuming structure as the basis of vital phenomena, in spite of the negative testimony of our imperfect microscopes. That function presupposes structure is now an accepted axiom, and we need only extend Brücke's method of

* C. O. Whitman. The Inadequacy of the Cell Theory of Development. Biological Lectures, 1893.