Development (biology)

“ Development is primarily central. All organic forms of which the entire history is known, set out with a symmetrical arrangement of parts round a centre. In organisms of the lowest grade no other mode of arrangement is ever definitely established; and in the highest organisms central development, though subordinate to another mode of development, continues to be habitually shown in the changes of minute structure. ”

“ This comparison between the course of development, in any creature, and the course of development in all other creatures—this arrival at the conclusion that the course of development in each, at first the same as in all others, becomes stage by stage differentiated from the courses in all others, brings us within view of an allied conclusion. If we contemplate the successive stages passed through by any higher organism, and observe the relation between it and its environment at each of these stages ”

“ So that throughout physiological development, as in evolution at large, the multiplication of effects has been a factor constantly at work, and working more actively as the development has advanced. The secondary changes wrought by each primary change, have necessarily become more numerous in proportion as organisms have become more complex. And every increased multiplication of effects, further differentiating the organism and, by consequence, further integrating it, has prepared the way for still higher differentiations and integrations similarly caused. ”

“ The matter will be simplified by two general observations—first, that the growth of biology is owing to concurrent progress in three fields of research, concerned, respectively, with the structure or architecture of living beings, their development, and their physiology. We recognize also a parallel advance in the systematic classification of animals and plants, and we note, furthermore, that [Pg 6] the idea of evolution permeates the whole. ”

“ Darwin's brief exposition, the belief in a progressive genesis of organisms is joined with an interpretation having considerable definiteness and coherence. In the space of ten pages he not only indicates several of the leading classes of facts which support the hypothesis of development, but he does something towards suggesting the process of development. ”

“ If all organisms have arisen by Evolution, it is of course not to be expected that such several modes of development can be absolutely demarcated: we are sure to find them united by transitional modes. But premising that a classification of modes can but approximately represent the facts, we shall find our general conceptions of Development aided by one. ”

“ Premonitions of Biological Evolution.
The eighteenth century had done much to impress the minds of men with an orderly development in sun and planets (Kant and Laplace) , in the institutions of human societies (Montesquieu) , and in the moral aspirations of mankind (Lessing) . Many bold attempts had been made to trace a like orderly development in the physical life of plants and animals (Buffon, Erasmus Darwin, etc.) ”

“ For if we follow out in thought the implications—if we conceive the germs of all kinds of organisms simultaneously developing, and imagine that after taking their first step together, at the second step one half of the vast multitude diverges from the other half; if, at the next step, we mentally watch the parts of each great assemblage beginning to take two or more routes of development ”

“ Thus, as the idea of development is not that of mere increase of size, but that of progress from what is not yet distinguished, but which potentially contains the distinction in itself, to the actually distinct,—it is clear, that the less an organ is developed, so much the more does it approach the type, and that, during its development, it more and more acquires peculiarities. ”

“ This is growth and reproduction in their simplest forms; and from such a simple beginning it is possible to conceive a series of slight modifications of composition, and of internal and external forces, which should ultimately lead to the development of more complex organisms. The LIFE of such an organism may, perhaps, be nothing added to it, but merely the name we give to the result of a balance of internal and external forces in maintaining the permanence of the form and structure of the individual. ”

“ The great leading use of this argument is in inducing us to regard the development of the individual animal as the precise equivalent of the series of changes by which the species was developed in the course of geological time. These two kinds of development are distinguished by appropriate names. Ontogenesis is the embryonic development of the individual animal, and is, of course, a short process, depending on the production of a germ by a parent animal or parent pair, and the further growth of this germ in connection more or less with the parent or with provision made by it. ”

“ Now, if the theory of evolution is true, what should we expect to happen when these germ-cells are fertilized, and so enter upon their severally distinct processes of development? Assuredly we should expect to find that the higher organisms pass through the same phases of development as the lower organisms, up to the time when their higher characters begin to become apparent. ”

“ By palingenetic processes, or embryonic recapitulations, we understand all those phenomena in the development of the individual which are transmitted from one generation to another by heredity, and which, on that account, allow us to draw direct inferences as to corresponding structures in the development of the species. ”

“ Thus the principle of natural selection is THE SELECTION OF THE BEST FOR REPRODUCTION, whether the "best" refers to the whole constitution, to one or more parts of the organism, or to one or more stages of development. Every organ, every part, every character of an animal, fertility and intelligence included, must be improved in this manner, and be gradually brought up in the course of generations to its highest attainable state of perfection. ”

“ All created organisms, during certain periods of their existence, are capable of development. The conditions being varied, plants and animals, including that strangely-constituted vertebrate, man, suddenly or by graduation, but not less surely, expand and change, or decrease and degenerate, as the case may be. ”

“ The individual development, or the ontogenesis of every single organism, from the egg to the complete form is nothing but a growth attended by a series of diverging and progressive changes. This applies equally to animals, plants, and protista. If, for example, we consider the ontogeny of any mammal, of man, of an ape, or of a pouched animal, or if we follow the individual development of any other vertebrate animal of another class, we everywhere find essentially the same phenomena. ”

“ 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. ”

“ Pathology, therefore, is really nothing else than modified physiology. Physiological development is the evolution of organs and the growth of organisms under the impression of natural external conditions; pathological development is the evolution of organs and the growth of organisms under the impression of unnatural external conditions. Adaptation of structure to function—of organisms to surrounding media—is the designed conservative purpose of both kinds of growth. ”

“ All we aim to show, is, that given a germ possessing these mysterious properties, the evolution of an organism from it, 46 probably depends upon that multiplication of effects which we have seen to be the cause of progress in general, so far as we have yet traced it.
When, leaving the development of single plants and animals, we pass to that of the Earth's flora and fauna, the course of our argument again becomes clear and simple. ”

“ 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? ”

“ These are the functions and capacities of living organisms in general, and in particular those of variation and inheritance, of development and self-differentiation. What is, and whence comes this mysterious power of the organism to build itself up from the smallest beginnings, from the germ? And the equally mysterious power of faithfully repeating the type of its ancestors? And, again, of varying and becoming different from its ancestors? Even the “mechanical” theory of selection is forced to presuppose the secret of life. ”

“ What is the place held in evolution by 'natural selection'? What, if any, the place held by use-inheritance? Is the factor of 'mimicry,' supported by Darwin, an important or even real factor in evolution? What is to be the issue of the controversy between the biologist and the physicist on the question of the time required for organic development? Are we to suppose that organic development at the beginning proceeded very much more rapidly than at a later stage? ”

“ The evolutionary process has been active not only in morphological differentiation, that is, in the development of new forms, but also in physiological differentiation, that is, in the development of specialised mechanisms for the performance of the various vital functions. ”

“ By development I do not merely mean their growth in the minds of those who first advanced them, but that larger development which consists in their subsequent good or evil fortunes—in their reception, favourable or otherwise, by those to whom they were presented. This is to an idea what its surroundings are to an organism, and throws much the same light upon it that knowledge of the conditions under which an organism lives throws upon the organism itself. ”

“ It is, for instance, a characteristic of the evolution of individual organisms under the laws of animal growth, as well as of that of the several species of animals under the influence of the struggle for existence and the law of the "survival of the fittest," that development takes place "spontaneously"—that is to say, is the incidental result of actions not consciously undertaken with that object in view. This is evidently so in the growth of an individual animal, and it is no less certainly so in the development of species. ”

“ In developing our conception we must discard from consideration the complexities that arise from the organization of the higher living bodies, the differences between one living animal and another, or between plant and animal. Such differentiations and integrations of living bodies are the subject-matter of discussions on evolution; some will see in the play of circumambient media, natural or supernatural, on the simplest forms of living matter, sufficient explanation of the development of such matter into the highest forms of living organisms ”

“ Physiology is also the first science in which we recognise the influence of habit—the tendency of something to happen again, merely because it has happened before. From physiology, too, we get our clearest notion of what is meant by development, or evolution. The growth of a plant or animal from the first germ is the typical specimen of a phenomenon which rules through the whole course of the history of man and society—increase of function, through expansion and differentiation of structure by internal forces. ”

“ And the results of these embryological investigations are in complete harmony with the requirements of the doctrine of evolution. The first beginnings of all the higher forms of animal life are similar, and however diverse their adult conditions, they start from a common foundation. Moreover, the process of development of the animal or the plant from its primary egg, or germ, is a true process of evolution--a progress from almost formless to more or less highly organised matter, in virtue of the properties inherent in that matter. ”

“ In 1852 he wrote an essay on The Development Hypothesis, in which he definitely stated his belief that the differentiation of species, like the differentiation within a single organism, was the result of development. ”

“ If we compare the factors essential for the different stages in development, we see that the question always resolves itself into one of modification of similar conditions common to all life-processes. We should rather have inferred that there exist specific external stimuli for each developmental stage, for instance, certain chemical agencies. Experiments hitherto made support the conclusion that QUANTITATIVE alterations in the general conditions of life produce different types of development. ”

“ There is a sense indeed in which creation by evolution or development is taking place for ever around us. Development is evolution beginning at a fixed point; evolution is only development ab initio.
Every spore that unfolds itself into a fern, every seed that gives origin to another tenant of the forest, every egg that peoples earth or air or sea with another life, is an instance of development. It is only abbreviated evolution; evolution which gives a written record of its laws in every stage, down to the primitive ovum. There the record of the organism in itself ceases. ”