Host (biology)

“ First, the evolution of the animal kingdom means essentially the evolution of the host, for that is what forms the individual; secondly, as the host is composed of a syncytium, the common factor of whose elements is the neural moiety, it follows that the tissue of central importance for the evolution of the host must be, as indeed it is, the nervous system. ”

“ In each of these groups there are fleas which have a habitat preference, that is to say, the flea lives on the host when the host lives in a given habitat, but is absent when the host lives in another habitat. ”

“ The evidence seems to show that the nodule organisms form a group that is in a state of divergent specialisation to life in different host-plants, and that this specialisation has reached different degrees with different hosts. ”

“ The fact that the cells which are out of harmony with the body are dependent upon nutritive material of the most varied nature for the opportunity of extending their existence, and more particularly for maintaining their species, gives us an insight into the kind of influence exercised by these parasites on the host. ”

“ In the last category are found those which need assistance all their lives; as soon as they have penetrated into the body of their host, they never remove again, and the lodging which they have chosen serves them both as a cradle and a tomb. ”

“ As adults they live as internal blood feeders in various birds, reptiles and mammals, especially in the nasal and respiratory passages. The larval stage occurs in another host in an encysted condition; this host is usually an animal preyed upon by the species in which the sexual forms are found. The larvæ bore through the walls of the host’s stomach and enter liver and spleen or brain, where they encyst; here they grow until they assume almost the appearance of the adult. ”

“ The question is more difficult than it appears at first sight, and we must remember that, complex as are the factors working on the host, they are equally complex in their actions on a parasite attacking the host, whence the resulting disease becomes indeed a tangled problem of natural selection. ”

“ The egg may be laid and the embryonic development passed through, outside the host, but the parasite on hatching may enter the host, so that the post-embryonic development is passed in the lymph of the host. ”

“ We have to regard the physical influences, varying from year to year, as now producing an extension or restriction of the habitat in this direction and now in that, and as producing secondary extensions and restrictions by their effects on other kinds of organisms. We have to regard the distribution of each species as affected not only by causes which favour multiplication of prey or of enemies within its own area, but also by causes which produce such results in neighbouring areas. ”

“ The physiological adaptations of parasites are notable, especially in cases where the hosts are warm-blooded. The parasites tend to become so specialized as to be peculiar to particular hosts; ectoparasites frequently differ from species to species of host, and the flea of one mammal, for instance, may rapidly die if it be transferred to another although similar host. ”

“ In the first place it is assumed that the fragment of matter out of which each organism arises consists of two portions—one of them, the germ-plasm, reserved within the generative organ of the incipient individual, representing in its components the structure of the species, and gives origin to the germs of future individuals ”

“ If it could use the body of a host, it would tend not to improve its own self-sufficient physical organism, as man has developed his body, but rather to perfect the ability to seize and to control its varied hosts."
I nodded eagerly, feeling hope stir and unfold. It was possible!
"However," the scientist said slowly, "it's rather a difficult step to accept the idea that such a parasitical life form could live in any host—even one from another planet and with a totally strange organism, such as man." ”

“ If, again, the circumstances of a species are modified by the intrusion of some foreign kind of plant or animal, it follows that since the intruders will probably not spread throughout its whole habitat, the species will, in one or more localities, remain unaffected by them. ”

“ A degree of comparative harmony may be gradually established between host and parasite, as is the case in wild animals. These have few diseases, the weak die, the resistant breed; they harbor, it is true, large numbers of parasites, but there is mutual adjustment between parasite and host. ”

“ Parasitic Habits.—The Trematodes with few exceptions select a vertebrate for their host. Speaking generally each species of parasite has a particular host, upon the blood of which it nourishes itself and matures its reproductive organs. ”

“ So, also, parasites obtaining protection from the tissues of their host lose their intrinsic protective mechanisms.
The reproduction of parasites offers many peculiarities, all of which are readily correlated with our generalization. A creature rigidly adapted to a special environment fails if it does not reach that environment, and hence species most successful in reproduction are able to afford the largest number of misses to secure a few hits and so to maintain existence. ”

“ Granted that each species has arisen by evolution from some other, this germ-cell which is observed in the body of the threadworm, must be regarded as part of what may well be called a stream of germ-plasm, that reaches back to the beginning of life in the world. ”

“ Not only the established truths of Biology, but the established truths of science in general, negative the supposition that organisms having structures definite enough to identify them as belonging to known genera and species, can be produced in the absence of germs derived from antecedent organisms of the same genera and species. ”

“ Such an association is an agent which transforms substance and energy, producing varied physiological conditions and responses in the continuous process of adjustment “which constitutes life.” The physiological needs and states of an association have as real existence in individual animals as have [Pg 6] similar needs in the cell or cells which compose the animal body. The mere statement of the facts of such relations is enough to make valid such a comparison.
For the associational aspect of ecology the German naturalist, Möbius, proposed in 1877 the term “biocœnosis.” ”

“ Thus the human body is the habitat of parasites, internal and external, animal and vegetal, numbering, if all are set down, between two and three dozen species; sundry of which are peculiar to Man, and many of which produce great suffering and not unfrequently death. What interpretation is to be put on these facts by those who espouse the hypothesis of special creations? According to this hypothesis, all these parasites were designed for their respective modes of life. They were endowed with constitutions fitting them to live by absorbing nutriment from the human body ”

“ The body is foliaceous, and they always apply themselves to their hosts with the long axis pointing forwards and parallel to that of the host, while on various parts of the under surface of the body are spines pointing backward which prevent the parasite being brushed off by the passage of the host through the water. ”

“ The parasites do not, as a rule, eat the structures of their host,—many of them, indeed, have no organs that would enable them to do this,—but they absorb the vegetable juices that, in a more or less altered state, form the lymph or so-called blood of the host. ”

“ The relations which obtain between parasites and their hosts are in all respects conditioned by their natural history; and without a detailed knowledge of the organisation, the development, and the mode of life of the different species, it is impossible to determine the nature and extent of the pathological conditions to which they give rise, and at the same time find means of protection against guests in every way so unwelcome. ”

“ It is otherwise when the species is removed to a habitat of different character, or when it is of mixed breed. In the one case its organs, partially out of harmony with the requirements of its new life, become partially out of harmony with one another; since, while one influence, say of climate, is but little changed, another influence, say of food, is much changed; and, consequently, the perturbed relations of the organs interfere with their original stable equilibrium. ”

“ One of these conclusions is that, in the formation of the germ which becomes developed into the fœtus, the male and female parent each contributes some cellular substance to the compound which constitutes that germ. We may safely infer this, because the individual animal becomes a union of characteristics belonging to both the parents, although the traits that are peculiar to one of the parents may be more or less marked in their different offspring, so that in one of the descendants the paternal and in another the maternal traits will predominate. ”

“ Whether it is a fact or not that somatic variations can induce corresponding variations in the germ-cells, [65] and thus be transmitted by heredity, it is certainly true that all heritages must come through the germ-cells. For this reason, it is clearly seen that so far as evolution is concerned the germ-cells are the factors of fundamental importance in organisms. Therefore, we may repeat that environment is the sum-total of the conditions of life that affect the germ-cells directly or indirectly. ”

“ It must not be forgotten that the germ is the entire organism and that it passes through development as the same individual; continuity of individuality is preserved throughout development. Therefore, if we discard determinant hypotheses and take our stand on a strictly physiological theory of development, it follows of necessity that the transmitted factors of heredity included in the organization of the germ cells must be factors in the development of the entire organism. ”

“ In the fecundated germ there are two groups of physiological units, presenting in their structures slight differences, so that by their fundamental resemblance they conspire to form an organism of the species to which the parents belong, and by their differences they give to this organism traits peculiar to each of the two parents. ”

“ It is clear that the organism that best uses its organs has, other things equal, the best chance of survival. It will therefore hand on to its offspring germinal matter with an inherent tendency to make vigorous use of its faculties.
Those who argue thus deny that the body-cells can in any way affect the germ-cells. To account for any continuous increase in faculty, they invoke variation and the selection of favourable varieties. What, then, we may now ask, is, on their view, the mode of origin of variations? ”

“ The chief characters of every living thing are unalterably fixed by heredity. Men do not gather grapes of thorns nor figs of thistles. Every living thing produces off-spring after its own kind, Men, horses, cattle; birds, reptiles, fishes; insects, mollusks, worms; polyps, sponges, micro-organisms,—all of the million known species of animals and plants differ from one another because of inherited peculiarities, because they have come from different kinds of germ cells. ”

“ In the case of the lower, imperfect organisms, among animals, especially in the case of the plant-like animals and worms, we very frequently find that in the interior of an individual composed of many cells, a small group of cells separates itself from those surrounding it, and that this small isolated group gradually develops itself into an individual, which becomes like the parent and sooner or later comes out of it.... The formation of germ buds is evidently but little different from real budding. ”

“ Individual traits are not transmitted from the hen to the egg, but they develop out of germinal factors which are carried along from cell to cell, and from generation to generation....”
“The germ is the undeveloped organism which forms the bond between successive generations; the person is the developed organism which arises from the germ under the influence of environmental conditions, the person develops and dies in each generation; the germ-plasm is the continuous stream of living substance which connects all generations. ”

“ Sooner or later they leave the organ inhabited by the parents, frequently reach the open, and after a shorter or longer period of free existence seek new hosts. During their free period, moreover, a considerable growth may be attained, or metamorphosis may take place, or even multiplication. In the exceptional cases in which the young remain within the same host, they nevertheless usually quit the organ inhabited by the parents. They likewise rarely attain maturity within the host inhabited by the parents, but only, as in other cases, after having gained access to fresh hosts. ”

“ Thus, without multiplying illustrations and without descending to genera and species, we see that, speaking generally, the successively smaller groups are distinguished from one another by traits of successively less importance, physiologically considered. The attributes possessed in common by the largest assemblages of organisms, are few in number but all-essential in kind. ”

“ The organism is thus considered as an agent which, transforming and utilizing substance and energy, produces a varied number of physiological conditions and forms of activity, which in turn furnish the basis for the constant process of response between the organism and its environment.
Aggregate Ecology.—The study of aggregate ecology is the investigation of the ecological development, relations, and responses of animals based upon hereditary or taxonomic units, as in a family community, or in genera, families, orders, etc. ”

“ A young organism arising by internal or external gemmation from a parent organism, passes gradually from a state in which it is an indistinguishable part of the parent organism to a state in which it is a separate organism of like structure with the parent. At what stage does it become an individual? ”