Life cycle

“ This Grand Cycle, which is but a pulsation or breath of the Eternal life, illustrates a truth which is repeated in its least and most minutely divided moment--that birth lies next to death, as water crystallizes at the freezing-point, and the plant blossoms at points most remote from the source of nutrition. ”

“ Has the duration and complexity of the life-cycle expanded or contracted since organisms first appeared on the earth? According to the current view, the life-cycle is continually being shortened at one end by the abbreviation of embryonic development and by the absorption of larval stages into the embryonic period, and lengthened at the other by the evolutionary creation of new adult phases. ”

“ And with this property of reacting to the environment goes the further property of undergoing a change which alters the relation of the organism to the old environment and places it in a new environment. If this reasoning is correct, it necessarily follows that this property must have been possessed by living matter at its first appearance on the earth. In other words living matter must always have presented a life-cycle, and the question arises what kind of modification has that cycle undergone? ”

“ Thus: it is a property of living matter to react in a remarkable way to external forces without undergoing destruction. The life-cycle, of which the embryonic and larval periods are a part, consists of the orderly interaction between the organism and its environment. The action of the environment produces certain morphological changes in the organism. These changes enable the organism to come into relation with new external forces, to move into what is practically a new environment, which in its turn produces further structural changes in the organism. ”

“ And thus it will be seen that human life, and all life, animal and vegetable, is but a mere incident in the vast operation of the physical laws of our solar system; the laws which clothe the earth with forests, that bring forth fruit, food and flowers in their season, and keep up the endless cycle of reproduction, age after age. ”

“ The life cycle of a species from among the higher plants or animals may be regarded as falling into three periods: (1) a period of isolation in the form of gametes, each a living unit incapable of further development without intimate association with another produced by the opposite sex; (2) a period of association in which two gametes become yoked together into a zygote and react upon one {6}another to give rise by a process of cell division to what we ordinarily term an individual with all its various attributes and properties ”

“ I have been careful to speak of the 'appearance' of cyclical evolution presented by living things; for, on critical examination, it will be found that the course of vegetable and of animal life is not exactly represented by the figure of a cycle which returns into itself. What actually happens, in all but the lowest organisms, is that one part of the growing germ (A) gives rise to tissues and organs ”

“ There is good reason why animals should have the periods of gestation, generation, and duration of life in certain cycles—I mean by cycle, a day, night, month, and year, and{524} the time which is described by these; also the motions of the moon—for these are the common origin of generation to all. ”

“ We may perhaps have a right to take up this position, but it is of no advantage to us to do so, because it leads us into the realm of pure fancy. Moreover, it assumes that an answer can be given to the question asked above—has the life-cycle of organisms contracted or expanded as the result of evolution? ”

“ Every cycle of evolution, in the individual life as in the collective life, bears within it the germs of the corresponding cycle of dissolution; and, inversely, the latter, by the decay of the form already worn out, prepares in the eternal laboratory new evolutions and new forms of life. ”

“ During this continuous life the idioplasm goes through a development of its own, just as an individual organism goes through a certain cycle of development during its individual life. This development consists in a constantly increasing complexity of structure and differentiation of function. ”

“ Under the influence of the life impulse, the old routine of matter—from compound to compound, from solid to fluid, from fluid to gaseous, from rock to soil, the cycle always ending where it began—is broken into, and cycles of a new order are instituted. ”

“ This remarkable state of affairs is accompanied by other peculiarities of physiology, with the result that the life-histories of successive generations become very diverse, and complex cycles of series of generations differing more or less from one another are passed through, the species finally returning to bi-sexual reproduction, and thus inaugurating another cycle of generations. ”

“ From very low forms up to the highest—in the animal no less than in the vegetable kingdom—the process of life presents the same appearance of cyclical evolution. Nay, we have but to cast our eyes over the rest of the world and cyclical change presents itself on all sides. ”

“ THE development of all living organisms is effected through certain energies and substances that are conducive to the growth and manifestation of life. It is the environment and physical surroundings that supply these life-sustaining factors to the organisms. And this physical universe is not only the feeder and sustainer of living beings, it is also the field of their activity as well as the abode in which they grow and reproduce themselves. Hence the action and reaction between the living organism and the environment regulate all the conditions of its life-history. ”

“ If an animal be resolved into its ultimate constituents in a period according to the surrounding circumstances, say, of four hours, of four months, of four years, or even of four thousand years, — for it is impossible to deny that there may be instances of all these periods during which the process has continued — those elements which assume the gaseous form mingle at once with the atmosphere and are taken up from it without delay by the ever-open mouths of vegetable life. ”

“ How great must have been the effects of such changes on every form of organic life! And it is to such as these we may perhaps trace those great changes of the animal world which have seemed to revolutionise it, and have led us to class one geological period as the age of reptiles, another as the age of fishes, and a third as the age of mammals. ”

“ And the constantly revolving circle of periodic changes of growth and decay, of life and death, of seed beginning and seed producing—like sunshine and shadow, light and darkness—are opposite points in the circle of electric force and physical life. The growing and dying life-forms intervene between these two points of rest and renewal, of attraction and repulsion, of positive organism and negative diffusion of matter. ”

“ This reduction of known biological phenomena to simpler terms, the lessening of the gap between inorganic and organic chemistry, and the formulation of the doctrine of the conservation of energy, have all prepared the way for a fourth step, the establishment of the inevitable theory of generatio spontanea sive equivoca, the spontaneous generation of the living, that is to say, the gradual evolution of the living from the not living. Since the earth, and with it the conditions under which alone life is possible, have had a beginning in time, life upon the earth must also have had a beginning. ”

“ Stated concretely in regard to living creatures, the general doctrine of organic evolution suggests, as we all know, that the plants and animals now around us—with all their fascinating complexities of structure and function, of life-history, behaviour, and inter-relations—are the natural and necessary results of long processes of growth and change, of elimination and survival, operative throughout practically countless ages ”

“ As plants and animals have gradually developed from lower to higher forms and the earth has been built gradually by formations at different periods of existence, by a comparison of the former development with the latter, that is, comparison with the earth, or inorganic, development to the life, or organic, development, we are enabled to get a comparative view of duration. ”

“ From this we may conclude that, but for the seasonal
energy-tides, the development of life upon the globe had gone
along very different lines from those actually followed. It is,
indeed, possible that the evolution of the larger organisms would
not have occurred; there would have been no vacant place for
their development, and a being so endowed as Man could hardly
[1] Weismann, _The Duration of Life._
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have been evolved. ”

“ Under the head of favorable conditions he enumerates variations in climate, temperature, the action of the environment, the diversity of local causes, change of habits, movement, action, variation in means of living, of preservation of life, of means of defence, and varying modes of reproduction. As the result of the action of these different factors, the faculties of animals, developed and strengthened by use, become diversified by the new habits, so that by slow degrees the new structures and organs thus arising become preserved and transmitted by heredity. ”

“ We propose in this essay to bring together, in as popular a manner as possible, some of the evidence in favor of this theory, endeavoring to show that the different plants and animals are linked together by transitional forms; that there has been a progress from the simple, lowly organized, to the highly complex forms of life; that the transitional stages through which a plant or an animal passes in the development from its primitive to its adult condition are permanently retained in the lower forms of life ”

“ If we appeal to physiology for the characteristics common to living beings, it will generally give us the following:—A structure or organization; a certain chemical composition which is that of living matter; a specific form; an evolution which in the earliest stage occasions the being to grow and develop until it is divided, and which in the highest stage includes one or more evolutive cycles with growth, the adult stage, senility, and death ”

“ To put it briefly, I consider that duration of life is really dependent upon adaptation to external conditions, that its length, whether longer or shorter, is governed by the needs of the species, and that it is determined by precisely the same mechanical process of regulation as that by which the structure and functions of an organism are adapted to its environment. ”

“ Activity is then not inimical to the organism, but is the appointed means by which the progressive and retrogressive metamorphoses must be carried out. In order that the process may go on harmoniously, or, in other words, that the body may be healthy, the whole organism, and every part of it in its own way, must have its period of productive activity and then also its period of rest in which it finds renewal of strength for another period of activity. Thus we have waking and sleep, inspiration and expiration of air. Periodicity is the law of life. ”

“ 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 be regarded as progressive or continuous adjustments of internal relations to external relations. So that not only does the definition, as thus expressed, comprehend all those activities, bodily and mental, which constitute our ordinary idea of Life ”

“ Upon these laws supervenes the law of effort by which organs are developed on various lines for various conditions of environment, and the result of this organic evolution leads to the survival of those best adapted. In animal life evolution is by affinity, adaptation, heterogeneity, survival, and effort. ”

“ The first sketches of an animal or a vegetable growth being begun under favourable conditions, the faculties of commencing life and of organic movement thus established have gradually developed little by little the various parts and organs, which in process of time have become diversified. ”

“ Hence, even on natural grounds, the most intelligible view of the history of the animal and vegetable kingdoms seems to be, that each period which is marked by a distinct collection of species forms a cycle; and that at the beginning of each such cycle a creative power was exerted, of a kind to which there was nothing at all analogous in the succeeding part of the same cycle. ”

“ The life of the individual consists of a series of processes which are cyclical. In the tree this is shown by the successive growths and deaths of series of leaves: in the animal by the development and exuviation of nails, hair, epidermis, &c.
The life of the species consists of a series of processes which are cyclical. ”

“ According to my conception, the physical and chemical changes which organisms experience during life through the action of the environment, through light or want of light, air, warmth, cold, water, moisture, food, etc., and which they transmit by heredity, are the primary elements in the production of the manifold variety of the organic world, and in the origin of species. ”

“ But there are organisms whose life-wave truly takes up the
periodicity of the Earth in its orbit. Thus the smaller animals
and plants, possessing less resources in themselves, die at the
approach of winter, propagating themselves by units which,
whether egg or seed, undergo a period of quiescence during the
season of want. In these quiescent units the energy of the
organism is potential, and the time-energy function is in
abeyance. ”