Biological development

“ On the other hand, we regard the development of the rudiments as depending upon motive forces or causes that are external to the germplasm of the ovum, but that none the less arise in orderly sequence throughout the course of the development. The causes we recognise are first, the continual changes in mutual relations that the cells undergo as they increase in number by division, and second, the influence of surrounding things upon the organism. ”

“ Such biological study determines attitude in no small degree. This is the chief justification for study of the reproductive processes in a series of animals and plants representing stages between the complex development of the highest animals which parallel human life and the lowest forms which the microscope reveals. ”

“ We must assume that each single part of the body at each developmental stage is, from the first, represented in the germ-cell as distinct particles of matter, which will reproduce each part of the body at its appropriate stage as their turn for development arrives. ”

“ 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. ”

“ The factors of evolution fall under two heads—origin and guidance. The origin of variations lies in mechanical stresses, and chemical or physical influences. Whether these act on the body (and are transmitted by inheritance) or only on the germ, is a question which divides biologists into two schools. In the latter case all variations are fortuitous; in the former the development of tissue-variations has been in direct response to the physical or chemical influences. ”

“ 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. ”

“ 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. ”

“ Heredity and Environment.—Unquestionably the factors, or causes, of development are to be found not merely in the germ but, also in the environment, not only in intrinsic but also in extrinsic forces; but it is equally certain that the directing and guiding factors of development are in the main intrinsic, and are present in the organization of the germ cells, while the environmental factors exercise chiefly a stimulating, inhibiting or modifying influence on development. ”

“ In practice, however, it can be successfully applied only to the lower and the higher forms of animal life. On the one hand, we could picture the growth of certain lowly organisms in the border land between the animal and vegetable worlds; on the other, we could portray the development of a child, or even the life changes of a human being from childhood to old age. ”

“ The next stimulus to such hypotheses was given by the progress of physiology, and especially by researches into the embryonic development of animals and plants. Here it was seen that there are homologies and likenesses of plan linking organisms with each other, and that in the course of their development the more complex creatures pass through stages corresponding to the adult condition of lower forms. ”

“ If the famous suggestion, that all animal and vegetable life results from a single vital germ, and that all the different animals and plants now existent were developed by a natural process of evolution from that germ, were a demonstrated truth, we should still be able to point to the evidence of intelligence displayed in the measured and progressive development, in those exquisite forms so different from what blind chance could produce, and in the manifest adaptation of surrounding circumstances to the living creature, and of the living creature to surrounding circumstances. ”

“ As life developed from simpler forms, each individual animal or plant became more complex, or carried a little further the process of growth. But the method of reproduction of new individuals remained fundamentally the same. Each individual began, like its ancestors, as a single-cell being. By the process of nutrition these single cells in each case would grow, divide, and produce various tissues and organs, but [1264] always repeating the general story of the development of the race. ”

“ 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. ”

“ It is a grand conception, this of the great physiologist, that every man, in the brief term of his prenatal development, should go through these successive changes, by which man has, in countless ages, been evolved from the primitive germ-cell; and it is triumphantly vindicated in The Evolution of Man. ”

“ The more healthy and perfect her organization, and the better the balance of all her organs, the sounder and the more perfect will be the development of her offspring. The health and life of the child demand it.
This theory of human increase derives strong evidence from an analogous law in the animal and vegetable kingdoms. It is well known that great improvements have been made within the present century in domestic animals, and this, too, by the application of physiological laws. ”

“ Modern embryology has completely negatived any such view as that of preformation, and as completely established that the evolution is not the unfolding of a miniature germ, but the growth and differentiation of primitively similar cell-elements. In different animals, as might be expected, the manner and course of development are different. ”

“ The variations in development, to which we here refer, take their origin in the germ-plasm which tends to repeat in each generation similar types of development. In other words the germ-plasm from which individuals spring is of such a nature that the embryos arising from it show in their development a recapitulation of the evolution of their particular species. ”

“ 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? ”

“ Darwin's hypothesis is not, so far as I am aware, inconsistent with any known biological fact; on the contrary, if admitted, the facts of Development, of Comparative Anatomy, of Geographical Distribution, and of Palæontology, become connected together, and exhibit a meaning such as they never possessed before; and I, for one, am fully convinced that if not precisely true, that hypothesis is as near an approximation to the truth as, for example, the Copernican hypothesis was to the true theory of the planetary motions. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ Finally, notwithstanding the difficulties I have mentioned, enough subservience to a fixed type on the part of pathological new formations has been observed, especially in cases where the new growth and its cause have been limited and simple, to warrant the assertion that pathological evolution in this respect is analogous with physiological development. ”

“ I have pointed out, in an earlier part of this chapter, that the history of all branches of science proves that they must attain a considerable stage of development before they yield practical 'fruits;' and this is eminently true of physiology. It is only within the present epoch, that physiology and chemistry have reached the point at which they could offer a scientific foundation to agriculture; and it is only within the present epoch, that zoology and physiology have yielded any very great aid to pathology and hygiene. ”

“ We know that rapidity of development is no less influenced by these two causes, and that, in general, the more subject an anatomical or physiological trait is to the influence of environment, the less definitely can we speak of a controlling influence of heredity, and the less are we justified in claiming that nature, not nurture, is the deciding element. ”

“ 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. ”

“ This immensely important process—which we may follow step by step, with our own eyes, any day in the embryological development of any animal or vegetable individual, and which as a rule is by no means considered with the reverence it deserves—informs us more surely and completely than all petrifactions could do as to the original palæontological development of all many-celled organisms, that is, of all higher animals and plants. ”

“ The cells are at first all alike in form and function, for all are globular in form, and each performs all the functions necessary for life. From this common point now commences development in different directions, which may be compared to a branching and rebranching, with more and more complex results, according as the animal is higher in the scale of organization and advances toward a state of maturity. ”

“ 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. ”

“ It is true that in the highest animals the germ cells after conjugating are still retained within the body of, and supplied with the nutriment by, one of the parents till development is considerably advanced, but in the sense in which I write they as truly lead a separate existence as the child whom the mother suckles. ”

“ As growth, organization, and reproduction are the functions of physical life, as soon as [Pg 28] the primordial germ began to live, it began to grow, to fashion organs however simple, for its nourishment and increase, and for the reproduction, in some way, of living forms like itself. ”

“ All parts of the organism are in a certain degree variable and capable of being determined by the strength and nature of the influences that affect them; and this capacity to respond conformably to functional stimulus must be regarded as the means which make possible the maintenance of a harmonious co-adaptation of parts in the course of the phyletic metamorphosis of a species. ”

“ It seems to me here indispensable to draw attention more closely to those infinitely important and interesting processes which accompany ontogenesis, or the individual development of organisms, and especially to that of vertebrate animals, man included. ”

“ In response to that condition, she substitutes for higher types of animal that development which we call embryonic, leaving for the lower type that which we call larval. Thus we see in the growth and formation of the higher animals and in the history of the comparative development of the animal kingdom, fresh illustrations of the great importance of the young type of cells. ”