Generation (biology)

“ The fact that living beings originate by generation from other living beings is one of the circumstances in their history which most completely differentiates them from minerals. This process of generation enters in various ways into the conception of biological species.
For example, it is a generally assumed axiom in biology that whatever proceeds from a living being by way of generation is of the same species as that from which it proceeds, whether the morphological differences between parent and offspring be great or small. ”

“ 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. ”

“ Have you ever met with any tangible and clear view of what takes place in generation, whether by seeds or buds, or how a long-lost character can possibly reappear; or how the male element can possibly affect the mother plant, or the mother animal, so that her future progeny are affected? ”

“ Considered in the light of this hypothesis, generation appears as a fact of disaggregation, occurring in an organism in process of equilibration: as a fact of disaggregation, which ever renews the evolution of the species, and which retards its equilibrium by multiplying the conditions under which the species may, under the influence of the incident forces of the environment, undergo a more perfect elaboration, the result of which shall be a better adaptation of the organism to its surroundings. ”

“ That the stream of generations flows on “by reproduction and genetic connection in the regular order of nature” is indeed a fact accordant with all observation, and to which there never has been known a solitary exception; but all observation proves that the regular order of nature in generation is confined within the limits of the species to which parents belong. ”

“ On considering this subject one should have imagined at first view, that it might have been easier for nature to have supported her progeny for ever in health and life, than to have perpetually reproduced them by the wonderful and mysterious process of generation. ”

“ Of the myriads of generations which such a process of development may have occupied, perhaps the first set moved with suctorial processes, while the second gradually adopted sedentary habits, and throughout the whole of the long series, each succeeding generation must have been almost exactly like its predecessor, and must always have consisted of individuals which possessed the characters of the species. ”

“ And on the whole the former sort will live longer, bear more offspring, and reproduce themselves more abundantly than the latter, and so generation by generation the average of the species will change in the favourable direction. ”

“ Here, then, we find a succession of progeny for several generations, and all quite unlike one another, yet without any immediate parental agency. Why is it not an example of spontaneous generation? and why may not new species be produced in this manner? ”

“ VIII. The Factors of Organic Evolution.
The animal kingdom represents one of the two grand branches of the organic universe. It has been evolved—evolved in a manner as simple and straightforward as it is revolting. It has all been brought about by partiality or selection. Generations of beings have come into existence. The individual members of each generation have differed from each other—differed in size, strength, speed, colour, shape, sagacity, luck, and likelihood of life. No two beings, not even those born from the same womb, are in all respects identical. ”

“ The idea held by him, in common with most other thinkers of his period, is still held more or less unconsciously by those who have not given particular attention to the subject. Generation is conceived as a direct chain: the body produces the germ-cell which produces another body which in turn produces another germ-cell, and so on. ”

“ In all the changes of structure which regularly supervene during old age, we see the effects of deteriorated growth, and not of true development.
In the so-called process of alternate generation many individuals are generated asexually during very early or later stages of development. ”

“ Generation being the only means of ascertaining the limits to which varieties may extend, species should be defined, the reunion of individuals descended from one another, or from common parents, or from such as resemble them as closely as they resemble each other ”

“ While admitting that generation, sexual or asexual, is the mode of reproduction found among animals possessed of complex structure, the partisans of spontaneous generation held, on the strength of their experiments, that certain very low organisms might be developed spontaneously, without specific germs, in infusions of organic substances. ”

“ 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. ”

“ But during the act of asexual production or subsequently to it, they often advance in organisation, as we see in the many cases of "alternate generation." In thus speaking of alternate generation, I follow those naturalists who look at the process as essentially one of internal budding or of fissiparous generation. ”

“ In the question of spontaneous generation, or archigony, which we can now answer more definitely, it must be borne in mind that by this conception we understand generally the non-parental generation of an organic individual, the339 origin of an organism independent of a parental or producing organism. ”

“ Indeed, as will be seen at the proper time, the mysteries of daily occurrence, such as growth, budding, and reproduction, are as great as any which beset the passage from form to form, from species to species. No one who has thought of it sufficiently can deny that the ordinary facts of generation, as the necessity of the union of two cells of nearly-spent vitality to produce a third cell, or a brood of cells, endowed with primeval vigor, present difficulties more unsurpassable than the origin of the germs of living things from inorganic matter ”

“ 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 science of eugenics is the natural result of the spread and acceptance of organic evolution, following the publication of Darwin's work on The Origin of Species by Means of Natural Selection, in 1859. It took a generation for his ideas to win the day; but then they revolutionized the intellectual life of the civilized world. Man came to realize that the course of nature is regular ”

“ 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." ”

“ The process of generation is still involved in impenetrable obscurity, conjectures may nevertheless be formed concerning some of its circumstances. First, the eggs of fish and frogs are impregnated, after they leave the body of the female; because they are deposited in a fluid, and are not therefore covered with a hard shell. ”

“ Darwin calls “natural selection.” Nature, so to speak, selects the best individuals out of each generation to live. And not only so, but as these favoured individuals transmit their favourable qualities to their offspring, according to the fixed pg. 4laws of heredity, it follows that the individuals composing each successive generation have a general tendency to be better suited to their surroundings than were their forefathers. ”

“ In other words, natural selection, in picking out of each generation those individual organisms which are by their congenital characters best suited to their surrounding conditions of life, is thereby picking out those peculiar combinations or variations of germ-plasm, which, when expanded into a resulting organism, give that organism the best chance in its struggle for existence. ”

“ But there is no good evidence that this ever follows in the course of a single generation. It appears, as in the case of general or indefinite variability, that several generations must be subjected to changed habits for any appreciable result. ”

“ Darwin, that where, by selection through successive generations, a part has been increased or decreased, its reaction upon other parts entails changes in them. This reaction is effected through the changes of function involved. If the changes of structure produced by such changes of function, are inheritable, then the re-adjustment of parts throughout the organism, taking place generation after generation, maintains an approximate balance; but if not, then generation after generation the organism must get more and more out of gear, and tend to become unworkable. ”

“ For the theory rests on two main pillars, the transmission of characteristics from progenitor to progeny, and the introduction of minute variations in the progeny with each successive generation. Now, the former of these may be said to be well established, and we recognise it as a law of life that all plants and animals propagate their own kind. But the latter has, as yet, been hardly examined at all. Each new generation shows special slight variations. ”

“ And since full comprehension of it requires some understanding of the nature of the physical mechanism by which hereditary traits are handed on from generation to generation, a small amount of space is given to this phase. Then, that the reader may appreciate to their fullest extent the facts gathered concerning man, a review of the more significant principles of genetics as revealed through experiments in breeding plants and animals has been undertaken. ”

“ And further, since the offspring, while inheriting their parents' peculiarities, tend, to vary somewhat from them, there will, in the next generation, probably be some who have the peculiarity in a greater degree than their parents, and others who have it in a lesser degree, when, if the conditions remain the same, there will be such a new survival of the fit, and such a fresh elimination of the unfit, as will leave the survivors in the third generation with the peculiarity developed, in however slight a degree, more than it was developed in their grandparents ”

“ Generation after generation this decreased [474] function changes its component families simultaneously. Natural selection does not cover the case at all—has nothing to do with it. And the like happens in multitudinous other cases. Every species spreading into a new habitat, coming in contact with new food, exposed to a different temperature, to a drier or moister air, to a more irregular surface, to a new soil, &c., &c., has its members one and all subject to various changed actions, which influence its muscular, vascular, respiratory, digestive, and other systems of organs. ”

“ We must construct a pedigree for the different classes or types of animals through which, by direct or collateral evidence, we can connect the different organisms together, so as to warrant the belief that by the ordinary process of generation these animals of widely different organizations have been successfully developed out of each other, life from life, organisms from organisms. ”

“ The innate ability is handed down from generation to generation, and, in so far as it contributes to success, is fostered and developed by selection; and the modifications of behaviour to which it leads, since the results of prior process in the parent persist as the basis and starting-point of subsequent process in the offspring may in a sense also be said to be handed down. ”

“ We have already seen how sexual generation produces variation of offspring and how this furnishes material for natural selection. As soon, therefore, as this form of generation was evolved, this higher factor came into operation, and immediately, as it were, assumed control of evolution, and the previous factors became subordinate though still underlying, conditioning, modifying the higher. The result was an immediate increase in the speed and in the diversity of evolution. ”

“ Heredity and Evolution. Charles Darwin.—There is still another important line of investigation in biology that we have not mentioned. This is the doctrine of evolution and the allied discoveries along the line of heredity. The development or evolution of plants and animals from simpler forms to the many and present complex forms of life have a practical bearing on the betterment of plants and animals, including man himself. The one name indelibly associated with the word evolution is that of Charles Darwin. ”

“ It is only from parent to offspring, through heredity and accumulation, that the living being becomes modified, and by degrees harmonises with the conditions of life. When, however, we are no longer studying an animal, plant, or a bird, which has the faculty of yearly reproduction, but species or races of a more tardy reproduction, we must remember that it is necessary to reckon by generations, and not by years. ”