Peter Chalmers Mitchell

“ PARASITISM, in biology, the condition of an organism which obtains its nourishment wholly or partially from the body of another living organism, and which usually brings about extensive modifications in both guest and host, a phenomenon widespread amongst animals and plants. ”

“ No doubt a large amount of variation is truly indefinite, so that many meaningless or useless variations arise, and in one sense it is a mere coincidence if a particular variation turn out to be useful. But there are several directions in which the field of variation appears to be not only limited but defined in a certain direction. Obviously variations depend on the constitution of the varying organism; a modification, whether it be large or small, is a modification of an already definite and limited structure. ”

“ Empirical Study of Heredity.—The fundamental basis of heredity is the separation of a mass from the parent (germ-plasm) which under certain conditions grows into an individual resembling the parent. The goal of the study of heredity will be reached only when all the phenomena can be referred to the nature of the germ-plasm and of its relations to the conditions under which it grows, but we have seen how far our knowledge is from any attempt at such references. ”

“ When we try to answer the question why the segments of an organism resemble one another, whether they remain in association to form a segmented animal, or break into different animals, we come to the conclusion, which at least is on the way to an answer, that it is because they are formed from pieces of the same protoplasm, growing under similar conditions. It is apparently a fundamental property of protoplasm to be able to multiply by division into parts, the properties of which are similar to each other and to those of the parent. ”

“ The real distinction between living matter and inorganic matter is chemical. Living substance always contains proteid, and although we know that proteid contains only common inorganic elements, we know neither how these are combined to form proteid, nor any way in which proteid can be brought into existence except in the presence of previously existing proteid. The central position of the problem of life lies in the chemistry of proteid, and until that has been fully explored, we are unable to say that there is any problem of life behind the problem of proteid. ”

“ We have still to admit with Darwin that it is difficult or impossible to assign utility to all the characters that distinguish species, and particularly to those characters by which systematists identify species. The modern tendency for a more complete and detailed separation of individual forms into specific and sub-specific groups, and the immensely larger range of material at the disposal of systematic experts, have combined to make it increasingly difficult to imagine conditions of the environment under which the species of systematists would have been produced by selection. ”

“ If we reflect on the multitude and complexity of such actions and reactions in operation from the youngest stages to the end of the life of each individual, we cannot be surprised at any correlation. Change in the size of any part or organ, however it may have been produced, must bring with it many others changes, directly or indirectly. A difference in calibre, elasticity or branching of a blood vessel, the smallest variation in a nerve or group of vessel-cells, any anatomical or physiological divergence, is reflected throughout the organism. ”

“ Origin of Parasitism.—Now that the theory of spontaneous generation has been disproved, the problem of parasitism is no more than detection of the various causes which may have led organisms to change their environment. Every kind of parasite has relations more or less closely akin which have not acquired the parasitic habit, and every gradation exists between temporary and permanent parasites, between creatures that have been only slightly modified and those that have been profoundly modified in relation to this habit. ”

“ Even horticulturists, who have been much more successful than those who deal with animals, are still far from being able to predict the result of their selections and crossings. None the less it may be stated definitely that such prediction is already so nearly within the power of the practical breeder that it would be a waste of time to give a summary of the existing rule-of-thumb methods. The art of breeding is so immediately destined to become a science of breeding that existing knowledge and conceptions must be dismissed as of no more than historical interest. ”

“ The primary distinction of sex resides in the essential organs of reproduction (q.v.) . An organism that contains the germinal tissue or mass of tissue known as the testis, and producing the sexual cells known as spermatozoa, is a male; an organism containing the tissue which produces ova is known as a female; one producing both ova and spermatozoa is a true hermaphrodite; and one producing neither, if it belong to the sexual generation, is known as a neuter, although neuters are for the most part incomplete females. ”

“ Darwin himself showed that different species in a genus, or varieties in a species, tended to show parallel variations, whilst comparative anatomy has made known a multitude of cases where allied series of animals or plants show successive stages of parallel but independent variations of important organs and functions. The phenomena of convergence are to some extent other instances of the same kind and supply evidence that organisms, so to say, fall into grooves, that their possibilities of change are defined and limited by their past history. ”

“ 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 ”

“ Saprophytes, organisms that live on organic matter, are merely parasites of the dead, whilst all animals derive their nourishment from the bodies of plants, either directly or indirectly through one or more sets of other animals. It is plain, therefore, that if parasitism is to be employed as a scientific term it must connote something more than mere dependence on another living organism for nutrition. ”

“ VARIATION AND SELECTION, in biology. Since the publication in 1859 of Charles Darwin's Origin of Species, the theory of evolution of animals and plants (see Evolution) has rested on a linking of the conceptions of variation and selection. Living organisms vary, that is to say, no two individuals are exactly alike; the death-rate and the multiplication-rate are to a certain extent selective, that is to say, on the average, in the long run, they favour certain variations and oppress other variations. Co-operation of the two factors appears to supply a causal theory of the occurrence of evolution ”

“ In sexless reproduction one parent is enough; in sexual reproduction equivalent masses of germ-plasm from each parent combine to form the new individual. The germ-plasm resides in the nucleus of cells, and Weismann identifies it with the nuclear material named chromatin. Like ordinary protoplasm, of which the bulk of cell bodies is composed, germ-plasm is a living material, capable of growing in bulk without alteration of structure when it is supplied with appropriate food. ”

“ Many animals of great zoological interest, from their nocturnal habits, or natural disposition, display themselves so seldom that their possession is valueless from the point of view of the public, whilst closely allied species are not distinguished except by trained observers. If the object of a collection is simply to provide a hardy and popular exhibition, it is neither difficult nor very costly to get together and to maintain. ”

“ Darwin certainly was impressed with the view that natural selection and variation together formed a mechanism, the central product of which was adaptation. From the Butlerian side, too, came the most urgent opposition to Darwinism. How is it possible, it was said, that fortuitous variations can furnish the material for the precise and balanced adaptations that all nature reveals? Selection cannot create the materials on which it is supposed to operate; the beginnings of new organs, the initial stages of new functions cannot be supposed to have been useful. ”

“ Among birds and mammals, the secondary sexual characters of one sex, such as size, pattern or colour, weapons or habits, may appear in animals with the gonads of the other sex, in every degree of development, reaching to an apparently complete reversal. In many cases these abnormal occurrences are associated with arrest of the functional activity of the primary organs of sex, by disease, accident, or decay, and the failure of the necessary stimulus would certainly serve to explain cases where the apparent reversal is no more than the suppression of a specialization in one direction. ”

“ The sterility which so generally attends the crossing of two specific forms is to be distinguished as of two kinds, which, although often confounded by naturalists, are in reality quite distinct. For the sterility may obtain between the two parent species when first crossed, or it may first assert itself in their hybrid progeny. In the latter case the hybrids, although possibly produced without any appearance of infertility on the part of their parent species, nevertheless prove more or less infertile among themselves, and also with members of either parent species. ”

“ To make the denotation of zoological names precise, exact workers are endeavouring to associate the conception of types with names, a process which can be made simple and definite with new work, but which presents great difficulties in the attempt to apply it to existing terms. Every family should have designated one of its genera as the type genus, every genus a type species and so forth. In the case of species or sub-species the type is a single specimen, either the only one before the author when writing his description, or one definitely selected by him, the others being paratypes. ”

“ Popular belief and some observations with regard to the breeding of domestic animals have led to the inference that the sex of the offspring tends to be that of the least vigorous parent, and such a theory, as it would appear to imply the existence of a natural law for rectifying the proportions of the sexes, has gained more attention than the facts supporting it would justify, and several unbiased observers have interpreted the events in the sense that the vigorous parent produces his or her own sex. ”

“ No doubt such domesticated species might revert, and it has been shown that many do revert when restored to wild conditions, but such reversion is natural if we reflect that the domestic varieties are under the guardianship of man and have been selected according to his whim and advantage. Comparing domesticated varieties with species and varieties in nature, Darwin showed that the distinction between varieties and species was chiefly a matter of opinion, and that the discovery of new linking forms often degraded species to varieties. ”

“ In the case of higher plants, the widespread occurrence of tissues with power of reproduction, the occurrence of budding in almost any part of the body in lower animals and in plants, and the widespread powers of regeneration of lost parts, are easily intelligible if every cell like the egg-cell has been formed by doubling division, and so contains the germinal material for every part of the organism, and thus, on the call of special conditions, can become a germ-cell again. ”

“ Darwin came to the conclusion that the sterility of crossed species must be due to some principle quite independent of natural selection. In his search for such a principle he brought together much evidence as to the instability of the reproductive system, pointing out in particular how frequently wild animals in captivity fail to breed, whereas some domesticated races have been so modified by confinement as to be fertile together although they are descended from species probably mutually infertile. ”

“ The inheritance of any character means the transmission in the germinal material of matter which, brought under the necessary external conditions, develops into the character of the parent. There is necessarily an acquired or epigenetic side to every character; but there is nothing in our knowledge of the actual processes to make necessary or even probable the supposition that the result of that factor in one generation appears in the germ-plasm of the subsequent generations, in those cases where an embryological development separates parent and offspring. ”

“ Comparison of living and lifeless organic matter presents the initial difficulty that we cannot draw an exact line between a living and a dead organism. The higher “warm-blooded” creatures appear to present the simplest case and in their life-history there seems to be a point at which we can say “that which was alive is now dead.” We judge from some major arrest of activity, as when the heart ceases to beat. ”

“ Both ideas, or both modes of expressing what is fundamentally the same idea, have this in common, that life is not a sum of the qualities of the chemical elements contained in protoplasm, but a function first of the peculiar architecture of the mixture, and then of the high complexity of the compounds contained in the mixture. The qualities of water are no sum of the qualities of oxygen and hydrogen, and still less can we expect to explain the qualities of life without regard to the immense complexity of the living substance. ”

“ With our present knowledge it is impossible to discriminate between variation that may or that may not be the material for the differentiation of species by scrutinizing either magnitude or probable causation. It is equally impossible to draw an exact line between variation induced by the environment and variation that may be termed intrinsic. Extrinsic and intrinsic factors are involved in every case, although there is a range from instances in which the external factor appears to be extreme to instances where the intrinsic factor is dominant. ”

“ The theory of breeds and breeding depends, in fact, on knowledge of variation, of modification by the environment, and of heredity. Any attempt to give an account of what actually has been done by man in establishing breeds would be little more than an imperfect summary of Darwin’s work. ”

“ In most respects his ideas were closely parallel with those of Darwin. He believed that species had been formed by means of natural selection. He insisted that the great powers of increase of all organisms led to a tremendous struggle for existence, and that variability extended to every part and organ of every organism; that the variability was large in amount in proportion to the size of the part, affected, and occurred in a considerable proportion of the individuals of those large and dominant species which might be supposed to be breaking up into new species. ”

“ If the first division of a reproductive cell set apart one mass to lie dormant for a time and ultimately to form the reproductive cells of the new generation, while the other mass, exactly of the same kind, developed directly into the new organism, then heredity would simply be a delayed case of what is called organic symmetry, the tendency of similar living material to develop in similar ways under the stimulus of similar external conditions. ”

“ The widest generalization that can be made regarding it is that parasitism tends towards a rigid adaptation to a relatively limited and stable environment, whilst free life tends towards a looser correspondence with a more varying environment. The summum bonum of a parasite is to reach and maintain existence in the limited conditions afforded by its host; the goal of the free-living organism is a varying or experimental fitness for varying surrounding conditions. ”

“ Differential fertility may be induced in either of two ways. Individuals may not be able to pair unless they possess a character which is absent, or insufficiently developed, in some members of the race. The kind of selection involved may then be measured by comparing those animals which pair with the general body of adults. This is what Darwin especially intended to denote by the term “sexual selection.” Or, again, individuals of certain character may be able to pair, but the fertility of their union may not be the same as that of unions between individuals with other characters. ”

“ Successful organisms, or well-adapted organisms, are those that have responded to the environment, whether by large or small variations, in suitable fashion. It is the character as acquired that affords the opportunity for selection, but the quality of responding to the environment so as to produce that character is transmitted. The conceptions of Weismann afford no ground for rejecting fluctuating variations from the materials for the production of species. ”