Ontogenesis

“ I shall endeavour in the following pages to show, step by step, how the whole of the embryonic phenomena form a massive chain of proof for the theory of evolution; for they can be explained in no other way. In thus appealing to the close causal connection between ontogenesis and phylogenesis, and taking our stand throughout on the biogenetic law, we shall be able to prove, stage by stage, from the facts of embryology, the evolution of man from the lower animals. ”

“ At this stage we leave for a while our analytical studies of ontogeny proper. We must not forget that typical ontogenesis is not the only form in which morphogenesis can occur: the organic form is able to restore disturbances of its organisation, and it certainly is to be regarded as one of the chief problems of analytical morphogenesis to discover the specific and real stimulus which calls forth the restoring processes. ”

“ The opposing views of to-day were in existence centuries ago, and they are known in the history of science as the theory of preformation or evolution, and the theory of epigenesis. That most of the great biologists of the seventeenth and eighteenth centuries were decided upholders of evolution was the natural result of the contemporary knowledge of facts. For they knew only the external signs of the process of development. [Pg 2] All they saw was the embryo becoming adult, the bud growing out into a blossom, as the result of a process in which nutrition transformed smaller to greater parts. ”

“ But development delivers its evidence relating to this matter with telling and impressive force. How can we doubt the possibility of an evolution of higher animals from ancestors as simple as Hydra and Amoeba when a frog and a cat, like all other complicated organisms, begin individual existence as single cells, and pass through gastrula stages? If we deny it, we contradict the evidence of our senses, for the development is actually accomplished by the transformation of a single cell into a double-walled sac, and of this into different and more intricate organic mechanisms. ”

“ But there is another essential element in ontogenic evolution. It is that the forces or causes of evolution are natural; that they reside in the thing developing and in the reacting environment. This we know is true of embryonic development; is it true also of the geologic succession of organic forms? It is true of ontogeny; is it true also of phylogeny? If not, then only by a metaphor can we call the process of change in the organic kingdom throughout geological history an evolution. ”

“ If we bear in mind that even the minutest variations of the biophors and determinants express nothing more or less than reactions to changed external conditions in the direction of adaptation, and that the same is true of each of the higher categories of vital units, whether they be called cell, tissue, organ, person, or corm, we see that the whole evolution of the forms of life upon the earth depends upon adaptations following each other in unbroken succession, and fitting into each other in the most complex way. ”

“ This is almost the same conception of ontogenetic development as that which has been held by embryologists who have not accepted the doctrine of evolution: for we have only to transfer the primary cause of development, from an unknown source within the organism, into the nuclear substance, in order to make the views identical. ”

“ The word "regeneration" has therefore in biology "come to mean, in general usage, not only the replacement of a lost part, but also the development of a new whole organism, or even a part of an organism, from a piece of an adult, or of an embryo, or of an egg." ”

“ These are Growth and Development. The mark of growth is that an organism, by assimilation from the outside world, becomes larger than it was. But in development it becomes different from what it was. The history of an embryo in the womb presents a succession of phenomena which, when one comes to realize them, almost stagger thought; for, while remaining the same thing all through, it is continually becoming a different class of thing—first two cells, then one cell, then a fish, a quadruped, ultimately a human being. ”

“ Phyletic growth, or the evolution of the organic world ever into higher and more complex forms, or at least into forms of different structure, is, as I have said, merely the sum of the processes of growth of the ancestors—together with the result of external influences on the forms during their development and their existence. This additional modification which the individuals as such undergo is—together with the influence of crossing—the very cause of the constantly progressing evolution. ”

“ The whole question of development is of great importance for our present subject. When an organ, the eye for instance, is metagenetically formed in a part of the body where during the previous stage of development no eye existed, we must look at it as a new and independent growth. The absolute independence of new and old structures, which correspond in structure and function, is still more obvious when several individuals are formed within a previous encasing form, as in the cases of alternate generation. ”

“ It results that the multiplied repetitions of these acts of organization strengthen, extend, develop, and even create the organs which are necessary. It is only necessary attentively to observe that which is everywhere occurring to convince ourselves of the well-grounded basis of this cause of organic developments and changes.
“Moreover, every change acquired in an organ by a habit of use sufficient to have produced it is then preserved by heredity (génération) if it is common to the individuals which, in fecundation, unite in the reproduction of their species. ”

“ The atomists will not admit the truth of this; they see in development nothing more than a process of the form-change and multiplication of cells. The full meaning of development escapes them, for they take no cognisance of the increasing complexity of the embryo, of the separating-out of tissues, of the moulding of organs, of the harmonious adaptation and adjustment of the parts to form a working whole. ”

“ This law, however, is less fundamental than the other two, and is, therefore, less conspicuous in the ontogenic than in the phylogenic series. It is conspicuous only in23 the later stages of ontogeny, and in other higher kinds of evolution, such as social evolution. For example, in the ontogenic development of the body and mind from childhood to manhood we have plainly successive culminations and declines of higher and higher functions. In bodily development we have culminating first the nutritive functions, then the reproductive and muscular, and last the cerebral. ”

“ Since it is the endeavour of the whole of modern biology to interpret life more and more through the interactions of the physical and chemical forces bound up with matter, development, too, comes within this aim, for development is an expression of life. We seek to understand the mechanism of life, and, as a part of that, the mechanism of development and of heredity which is closely associated with it. ”

“ Thus alternation of generations in cells exists, because the successive generations become more and more complicated at each succeeding period. Among these periods the ontogenetic period or ontogeny embraces all generations from one cell to the return of the exactly similar kind of cell. ”

“ For that process of evolution during which the organs of the embryo are fitted to their prospective functions, is the gradual or continuous adjustment of internal relations to external relations. Moreover, those structural modifications of the adult organism which, under change of climate, change of occupation, change of food, bring about some re-arrangement in the organic balance, may similarly {100}be regarded as progressive or continuous adjustments of internal relations to external relations. ”

“ It is a great merit of the modern tendency in evolution theory that it has demonstrated the importance of such internal influences. Although we are still far from being able to define the manner in which these influences operate, we may say so much, that it depends essentially on the nature and extent of the loss which parts are reproduced by the regenerating cells, and, also, on the position and direction of the injured surface from which the regeneration starts. ”

“ We cannot therefore avoid referring the processes of regeneration to particular 'regeneration-determinants,' which are contained in the germ-plasm and are handed on in ontogeny with the other determinants from cell-division to cell-division, till ultimately they reach the cells which are to respond, or may have to respond, to the stimulus of injury by some expression of their regenerative capacity. ”

“ But how are we to account for the operation of causes that have preserved a parallelism of development, along with the operation of causes that have produced the different modes of development, when all the species are supposed to be derived from a common ancestral stock, which first began to procreate and to develop its descendants in one and the same way? What are the facts which will enable us to say that the mammalian type arose out of the lower vertebrate types, when we compare the different modes of their embryologic development? ”

“ Professor Graham Kerr in his work on Embryology before mentioned says: “In early stages of Evolution, whether phylogenetic or ontogenetic, we may take it that vital impulses flitted hither and thither in an indefinite manner within the living substance and that one of the features of progressive evolution has been the gradual more and more precise defini­tion of the pathways of particular types of impulse, as well as the transmitting and receiving centres between which they pass. ”

“ Thus generations are generations of processes. The processes are assimilation, construction and destruction, growth of form and structure, vitality exhibited in endosmosis and exosmosis, and finally processes are repeated by heredity represented by parents and children. ”

“ What is development? Does it imply preformation or epigenesis? This perplexing question of biology has reappeared recently as a problem of the day. Of late years there have been set forth contradictory doctrines, each seeking to explain the process by which the fertilised egg-cell, an apparently simple beginning, gives rise to the adult organism, which often is exceedingly complicated, and which has the capacity of producing new beginnings like that from which it itself arose. ”

“ In the lowest forms of cell differentiation the cells of successive generations are all independent; the ontogenetic period consists of a cycle of generations of unicellular plants. Later the cell generations of an ontogeny are united by parts into plant individuals; the ontogenetic period consists of a cycle of multicellular and unicellular, or only of multicellular plant generations. If all the cell generations of an ontogenetic period have been united into a single individual, the successive plant generations are alike and alternation of generations has ceased. ”

“ According to Professor Oken, one of the ablest supporters of the development theory, “There are two kinds of generation in the world, the creation proper, and the propagation that is sequent thereon, or the original and secondary generation. Consequently, no organism has been created of larger size than an infusorial point. No organism is, or ever has been created, which is not microscopic. Whatever is large has not been created, but developed. Man has not been created, but developed.” ”

“ And now observe how, when we take this for the unit of composition, the metamorphoses which the phænogamic axis displays, are inferable from known laws of development. Embryology teaches us that arrest of development shows itself first in the absence of those parts that have arisen latest in the course of evolution; that if defect of nutrition causes an earlier arrest, parts that are of more ancient origin abort ”