Chapter IX: Part 9

The instruments of water-respiration which we call gills (_branchiæ_) are generally attenuated parts or processes of the outer skin or the inner gastric skin; hence we distinguish the two chief forms, external and internal gills. Both are richly provided with blood-vessels which bring the blood from the body for the purpose of aëration. Cutaneous or external gills are especially found in the vertebrates, in the form of threads, combs, leaves, pencils, tufts of feathers, etc., which are drawn out from the entoderm as local processes of the outer skin, and afford a wide surface for the interchange of gases between the body and the water. In the mollusca there are usually a pair of comb-shaped gills near the heart; in the articulates there are several pairs, repeated in the different segments of the body. Gastric or internal gills are peculiar to the vertebrates and the next-related tunicates, with a small group of the vermalia, the enteropneusta. In these the fore-gut or head-gut is converted into a gill-organ, the wall of which is pierced with gill-fissures; the water that has been taken in by the mouth passes away through the outer openings of these fissures. In the lower aquatic vertebrates (acrania, cyclostoma, and fishes) the gills are the sole organs of breathing; in the higher animals, that live in the air, they fall into disuse, and their place is taken by lungs. Nevertheless, heredity is so tenacious that we find from three to five pairs of rudimentary gill-clefts in the embryo right up to man, though they have long since ceased to have any function. This is one of the most interesting of the palingenetic facts that prove the descent of the amniotes (including man) from the fishes.

The group of the aquatic echinoderms has some very peculiar features of respiration. Their body possesses an extensive water-duct, which takes in the sea-water and returns it by special openings (skin-pores or madreporites). The many branches of these water-vessels or ambulacral vessels fill with water, especially the tiny feelers or feet, which extend from the skin in thousands; they serve at once for movement, feeling, and breathing. But many of the echinoderms have also special gills--the star-fish have small finger-shaped cutaneous gills on the back, the sea-urchins special leaf-shaped ambulacral gills, the sea-cucumbers internal gastric gills (tree-shaped branching internal folds of the rectum).

The organs of air-breathing are called, in general, lungs (_pulmones_). Like the organs of water-breathing, they are formed sometimes from the external and sometimes from the internal covering of the body. Cutaneous or external lungs are found in several groups of the vertebrates. Among the mollusks the land-dwelling lung-snails have acquired a lung-sac by change in the work of the gill cavity: among the articulata the lung-spiders and scorpions have two or more trachea-lungs; that is to say, cutaneous sacs, in which are enclosed fanwise a number of trachea-leaves. In the other air-breathing articulates (tracheata) we find, instead of these simple or branched, and often bushlike, air-tubes (_tracheæ_), which spread through the whole body and conduct the air direct to the tissues. They take the air from without by special air-holes in the skin (_stigmata_ and _spiracula_). The myriapods and insects generally have numbers of air-holes; the spiders only one or two, more rarely four, pairs. When these air-tube animals return to an aquatic life (as happens with the larvæ of various groups of insects), the outer air-holes close up, and new thread-shaped or leaf-shaped trachea-gills are formed, which take the air from the surrounding water by osmosis. The oldest and lowest tracheata are the primitive air-tube animals, or protracheata, and form the link between the older annelids and the myriapods. They have a number of clusters of short air-tubes distributed over the whole skin, and it is clear that these have been evolved from simple skin-glands by change of function.

Gastric or internal lungs are only found in the higher animals, to which we give the name of quadrupeds (or _tetrapoda_), the amphibia and amniotes, and their fishlike ancestors, the dipneusta. These internal lungs are sac-shaped folds of the fore-gut, formed originally from the swimming-bladder (_nectocystis_) of the fishes by change of function. This air-filled bladder, a sac-shaped appendage of the gullet, merely serves the purpose of a hydrostatic organ, by varying the specific weight, in the fishes. When the fish wishes to descend it contracts the bladder and becomes heavier; it rises to the top by inflating it again. The lungs were formed by the adaptation of the blood-vessels in the wall of the swimming-bladder to the interchange of gases. In the oldest living lung-fishes (_ceratodus_) it is still a simple sac (_monopneumones_=one-lunged); in the others the simple gullet-cavity divides early into a pair of sacs (_dipneumones_, two-lunged). The wind-pipe (_trachea_--not to be confused with the organ of the same name in the tracheata) is formed by the lengthening of their stalk and strengthening of it with cartilaginous rings. At the anterior end of the trachea we find already formed in the amphibia the larynx, the important organ of voice and speech.

The function of removing unusable matter is not less important to the organism than breathing. Just as breathing gets rid of the poisonous carbonic acid, so the kidneys remove fluid and solid excreta in the shape of urine; these are partly acid (uric acid, hippuric acid, etc.), partly alkaline (urea, guanine, etc.). In most of the cœlomaria special organs for removing these would be superfluous, as this is accomplished (like breathing) by the stream of water that is constantly passing through the whole body. But with the platodes we begin to find important excretory organs in the nephridia, a pair of simple and ramified canals which lie on either side of the gut, and open outward. These primitive renal canals are transmitted by the platodes to the vermalia, and by these to the higher stems of the cœlomaria. In the latter they generally open by special funnels into the inner body-cavity, which serves as first receptacle for the urine. Their outer opening sometimes (primarily) goes through the outer skin at the back (excretory pores), sometimes (secondarily) to the rectum, and so out through the anus. The oldest articulates, the annelids, have a pair of nephridia in each segment of the body; each renal canal, or segmental canal, consists of three sections, an inner funnel which opens into the body-cavity, a middle glandular section, and an external bladder that ejects the urine by contraction. The disposition of the renal system in the internally articulated vertebrates is very similar to this; but now complicated structures begin to appear, a pair of compact kidneys (_renes_), which are made up of a number of branching nephridia. Three generations of kidneys succeed each other, as phylogenetic stages of evolution--first the primary fore-kidneys (_protonephros_), in the middle the secondary primitive kidneys (_mesonephros_), and last the tertiary after-kidneys (_metanephros_). The latter are only reached in the three highest classes of vertebrates, reptiles, birds, and mammals. Mollusks also have a couple of compact kidneys. They are developed from a pair of nephridia, the funnels of which open internally into the heart-pouch (the remainder of the reduced body-cavity); at the back they open outward. The crustacea also have generally a pair of renal canals. On the other hand, the protracheata (the stem-forms of the air-tube animals) have segmental nephridia, a pair to each joint inherited from their annelid ancestors. The rest of the tracheata, the myriapods, spiders, and insects, have, instead of these, Malpighi tubes, funnel-shaped glands that arise from the entodermal rectum, sometimes one pair or less, sometimes a number in a cluster.

While most plants are purely plasmodomous, and most animals plasmophagous, there are nevertheless in both organic kingdoms a number of species (especially the lower) whose metabolism has assumed peculiar forms by their relations to other organisms. To this class belong especially the saprosites and parasites. By saprosites are understood those plants and animals which feed entirely or mostly on the corpses of other animals, or the decomposed matter which is unfit for the food of higher animals. Among the unicellular protists many of the bacteria, especially, belong to this class, and also many fungilla (_phycomycetes_); among the metaphyta the fungi (mycetes), and among the metazoa the sponges. I have already spoken of the many peculiarities of metabolism in the ubiquitous bacteria; while many of them cause putrefaction, they at the same time feed on the parts of other organisms which have died. The fungi feed for the most part on the decayed remains of plants and the products of putrefaction which accumulate on the ground. In this character of scavengers they play the same important part on land as the sponges do at the bottom of the sea. But a number of small groups of the higher plants and animals have, as a secondary habit, turned to saprositism. Among the metaphyta we have especially the monotropea (to which our native asparagus, _monotropa hypopitys_, belongs) and many orchids (_neottia_, _corallorhiza_). As they find their plasm directly in the decayed matter in the woods, they have lost their chlorophyll and green leaves. Among the metazoa many of the vermalia, and some of the higher animals, such as the rain-worm and many tube-dwelling annelids (the mud-eaters, _limicolæ_), etc., live on putrid matter. The organs which their nearest relatives use for obtaining, breaking up, and digesting food (eyes, jaws, teeth, digestive glands) have been entirely or mostly lost by these saprosites. Many of them form a transitional type to the parasites.

By parasites, in the narrower sense, we understand, in modern biology, only those organisms which live on others and derive their nourishment from them. They are numerous in all the chief divisions of the plant and animal kingdoms, and their modifications are of great interest in connection with evolution. No other circumstance has so profound an influence on the organism as adaptation to a parasitic existence. Moreover, there is no other section in which we can follow, step by step, the course of the degeneration which is caused, and show clearly the mechanical nature of the process. Hence the science of parasites--parasitology--is one of the soundest supports of the theory of descent, and provides an abundance of the most striking proofs of the much-contested inheritance of acquired characteristics.

Among the unicellular organisms, the bacteria are the most conspicuous instances of manifold adaptation to parasitic habits. As we count these unnucleated protozoa among the oldest and simplest organisms, and trace them directly by metasitism to the plasmodomous chromacea, it is very probable that they turned to parasitism very early in the history of life. Even a part of the monera (in which group we must place the bacteria on account of their lack of a nucleus) found it convenient and advantageous to prey on other protists and assimilate their plasm directly, instead of going through the laborious process of carbon assimilation themselves in the hereditary fashion. This is also true of the large class of the sporozoa or fungilla (_gregarinæ_, _coccidia_, etc.), real nucleated cells, which have adapted themselves in various ways to parasitic habits. Many of them live in the rectum, the cœlum, or other organs of the higher animals (the gregarinæ, especially in the articulates); others in the tissues (for instance, the sarcosporidia in the muscles of mammals, the coccidia and myxosporidia in the liver of vertebrates). A good many of them are "cell-parasites," and live inside the cells of other animals, which they destroy; such are the hœmosporidia, which destroy the blood-cells in man, and so cause intermittent fever.

Among the multicellular metaphyta it is particularly the fungi that have taken to parasitism in various ways. Many of them are, as is known, the most dangerous enemies of the higher animals and plants. The various species of fungi cause certain diseases by their poisonous (chemical) action on the tissues of their host. It is well known how our most important cultivated plants, the vine, potato, corn, coffee, etc., are threatened by fungoid diseases; and this is also true of many of the lower and higher animals. It is probable that the fungi have been evolved polyphyletically by metasitism from the algæ.

Among the higher metaphyta we find parasitism in many different families, especially orchids, rhinanthacea (_orobranche_, _lathraca_), convolvulacea (_cuscuta_), aristolochiacea, loranthacea (_viscum_, _loranthus_), rafflesiacea, etc. These various kinds of flowering-plants often grow to resemble each other by convergence (that is to say, by their common adaptation to parasitic life); they lose their green leaves, the plasmodomous chlorophyll of which is of no further use to them. Frequently rudimentary leaves are left on them in the form of colorless scales. For the purpose of clinging to the plants they live on, and penetrating into their tissues, they evolve special clinging apparatus (haustoria, suctorial cups, creepers). Their stalks and roots are also modified in a characteristic way. The whole productive force of these parasites is expended on their sexual organs; _rafflesia_ has the largest flowers there are, more than a yard in diameter.

Parasitism in the metazoa (in all groups) is even more frequent and interesting than in the metaphyta. The mollusks and echinoderms show the least disposition for it, and the platodes, vermalia, and articulates the most. Even among the gastræada, the common ancestral group of the metaphyta, we find parasites (kyemaria and gastremaria). The protection they find inside their hosts is probably the reason why these oldest of the metazoa have remained unchanged to the present day. Real parasites are not numerous among the sponges and cnidaria. But they are very numerous among the platodes. The suctorial worms (_trematodes_) live partly externally (as ectoparasites) on other animals and partly inside them (as endoparasites), and produce serious diseases in them. They have lost the vibratory coat of their free-living ancestors, the turbellaria, and acquired clinging apparatus instead. The tape-worms (_cestodes_), which live entirely in the interior of other animals, and are descended from the suctorial worms, have lost their gastro-canal; they are nourished by imbibition through the skin. The same degeneration is found in the itchworms (_acanthocephala_) among the vermalia, the parasitic snails (_entoconcha_) among the mollusks, and the root-crabs (_rhizocephala_) among the crustacea.

The class of crustacea affords the most numerous and most instructive examples of degeneration through parasitism, because in this class it is found polyphyletically in very different orders and families, and because their highly organized body shows every stage of degeneration together in the different organs. The free-living crustacea generally move about very rapidly and ingeniously; their numerous bones are well jointed and excellently adapted for the most varied methods of locomotion (running, swimming, climbing, digging, etc.); their organs of sense are highly developed. As these are no longer used when they take to parasitism, they atrophy and gradually disappear. The younger crustacea all proceed from the same characteristic form of the _nauplius_, and swim freely about; later, when they settle down to parasitic habits, their organs of sense and locomotion atrophy. As Fritz Müller-Desterro showed in his famous little work, _For Darwin_ (1864), forty years ago, the crustacea afford most luminous proofs of the theory of descent and selection, and of progressive heredity and the biogenetic law. These facts are the more important as the crab undergoes the same degeneration by parasitic habits in a number of different orders and families.

From parasitism we must entirely distinguish that intimate life-union of two different organisms which we called symbiosis or mutualism. Here we have an association of two living things for their mutual benefit, while the parasite lives entirely at the expense of his host. Symbiosis is found among the protista, being very wide-spread among the radiolaria. In the gelatinous envelope (_calymma_) which encloses the central capsule of their unicellular bodies we find a number of motionless yellow cells (_zooxanthella_) scattered. These are protophyta or (as it is said) "unicellular algæ" of the class of paulotomea (_palmellacea_). They receive protection and a home from the radiolaria, grow plasmodomously, and multiply by rapid segmentation. A large part of the starch-flour and the plasm which they form by carbon-assimilation goes as food directly to the radiolarium-host; the other part of the xanthella goes on growing and multiplying. Similar yellow zooxanthella or green zoochlorella are found as symbionta in the tissues of many animals. Our common fresh-water polyp (_hydra viridis_) owes its green color to the zoochlorella which live in great numbers on the ciliated cells of its entoderm (the digestive gut-epithelium). In general, however symbiosis is rarer in the metazoa than in the metaphyta. In the latter case it is the fundamental feature of a whole class of plants, the lichens. Each lichen consists of a plasmodomous plant (sometimes a protophyte, sometimes an alga) and a plasmophagous fungus. The latter affords a home, protection, and water to the green alga, which repays the service by providing food.

XI

REPRODUCTION

Reproduction and generation--Sexual and asexual reproduction--Superfluous growth--Monogony--Self-cleavage--Budding--Formation of spores--Amphigony--Ovum and sperm-cell--Hermaphrodite formation and separation of the sexes--Hermaphrodism and gonochorism of the cells--Monoclinism and diclinism--Monœcism and diœcism--Alternation of sex-division--Sexual glands of the histona--Hermaphroditic glands--Sexual ducts--Generative organs--Parthenogenesis--Pædogenesis--Metagenesis--Heterogenesis-- Strophogenesis--Hypogenesis--Hybridism--Generation of hybrids and the species--Graduation of forms of reproduction.

While nutrition secures the maintenance of the organic individual, reproduction insures that of the organic species, or the group of definite forms which we distinguish from others by the name "species." All individuals are more or less restricted in the duration of their lives, and die off after the lapse of a certain time. The succession of individuals, connected by reproduction and belonging to a species, makes it possible for the specific form itself to last for ages. In the end, however, the species is temporary; it has no "eternal life." After existing for a certain period, it either dies or is converted by modification into other forms.

The rise of new individuals by reproduction from parent organisms is a natural phenomenon with definite time-restriction. It cannot have continued from eternity on our planet, as the earth itself is not eternal, and even long after its formation was incapable of supporting organic life on its surface. This only became possible when the surface of the glowing planet had sufficiently cooled for liquid water to settle on it. Until this stage carbon could not enter into those combinations with other elements (oxygen, hydrogen, nitrogen, and sulphur) which led to the formation of plasm. As I intend to deal with this process of _archigony_, or spontaneous generation, in a special chapter, I leave it for the present, and confine myself to the study of _tocogony_, or parental generation.

* * * * *

The various forms of tocogony, or the reproduction of living things, are generally divided into two large groups; on the one hand there is the simple form of asexual generation (monogony), and on the other the complex form of sexual generation (amphigony). In asexual generation the action of one individual only is needed, this providing a product of transgressive (redundant) growth which develops into a new organism. In sexual generation it is necessary for two different individuals to unite in order to produce a new being from themselves. This amphigony (or _generatio digenea_) is the sole form of reproduction in man and most of the higher animals. But in many of the lower animals and most of the plants we find also asexual multiplication, or monogony, by cleavage or budding. In the lowest organisms, the monera and many of the protists, fungi, etc., the latter is the only form of propagation.

Strictly speaking, monogony is a universal life-process; even the ordinary cell-cleavage, on which depends the growth of the histona, is a cellular monogony. Hence historical biology must say that monogony is the older and more primitive form of parental generation, and that amphigony was secondarily developed from it. It is important to emphasize this because, not only some of the older writers, but even some recent ones, regard sexual generation as a universal function of organisms, and declare that it dates from the very beginning of organic life.

The complex and frequently very intricate phenomena of sexual generation, as we find them in the higher organisms, become intelligible to us when we compare them with the simpler forms of asexual generation at the lowest stages of life. We then learn that they are by no means unintelligible and supernatural marvels, but natural physiological processes, which, like all others, may be traced to the action of simple physical forces. The form of energy which lies at the root of all tocogony is _growth_ (_crescentia_). And as this phenomenon is also the cause, in the form of gravitation, of the formation of crystals and other inorganic individuals, we do away with another of the boundaries which people would establish between organic and inorganic nature. Reproduction is a kind of nutrition and growth of the organism beyond the individual standard, building up a part of it into a whole. This _limit_ of individual size is determined for each species by two factors--the inner constitution of the plasm, which is inherited, and the dependence on the outer environment, which controls adaptation. When this limit has been passed, the transgressive growth takes the form of reproduction. Every species of crystal has also a definite limit of growth; when this is passed, new crystal-individuals are formed in the mother-water on the old individual, which grows no further.

Asexual or monogenetic tocogony (also called "vegetative multiplication") is always effected by a single organic individual, and so must be traced to its transgressive growth. When this affects the entire body as a total growth, the whole dividing into two or more equal parts, we call the monogenetic process division (or segmentation). But when the growth is partial, and affects only a part of the individual, or when this special part separates from the generating organism in the form of a bud (_gemma_), the process is called budding or gemmation (_gemmatio_). Hence the essential difference between the two forms of generation is that in division the parent disappears in its partial products (children); these are of the same age and form. But in budding the generating parent retains its individuality; it is larger and older than the young bud. This important difference between division and gemmation, which is often overlooked, holds good both for protists (unicellulars) and histona (multicellulars). The fact that in division the individual as such is destroyed is a sufficient refutation of Weismann's theory of the immortality of the unicellulars. (See above, and also the _Riddle_, chapter xi.)

Reproduction by division is by far the most common of all forms of propagation. It is the normal form of monogony, not only in many of the protists, but also in the tissue-cells which compose the tissues of the histona. It is, moreover, the sole method of propagation for most of the monera, both chromacea and bacteria, which are in consequence often comprised under the title of "cleavage-plants" (_schizophyta_). Self-cleavage is also found among the higher multicellular organisms--namely, the cnidaria (polyps, medusæ). It usually takes the form of division into two parts (_dimidiatio_ or hemitomy), the body splitting into two equal halves. The plane of division is sometimes indefinite (fragmentary-cleavage), sometimes coincident with the long axis (length-cleavage), sometimes with the transverse axis, vertical to the long axis (transverse-cleavage), and less frequently with a diagonal axis (oblique-cleavage). When the segmentation of a cell proceeds so rapidly that the transverse-cleavage follows immediately on the length-cleavage, and the two are at length made to coincide, twofold division is changed into fourfold division. And when the process is repeated in quick succession, and the body falls at last into a number of small and equal pieces, we have manifold division (polytomy); as in the spore-formation of the sporozoa and rhizopoda, and in the embryonic sac of the phanerogams.

Asexual propagation by budding is chiefly distinguished from segmentation by the fact that the determining transgressive growth is only partial in the one and total in the other. The bud produced is, therefore, younger and smaller than the parent from which it issues; the latter may replace the lost part by regeneration and produce a number of buds simultaneously or successively without losing its individuality (whereas this is destroyed in division). Propagation by budding is rare among the protists, and more common among the histona--that is, with most of the tissue-plants and the lower, stock-forming, tissue-animals (cœlenteria and vermalia). Most stocks (cormi) are formed by a sprout or person shooting out buds which remain united to it. The layer and shoots of tissue-plants are detached buds. The two chief kinds of gemmation are terminal and lateral. Terminal budding takes place at the end of the long axis, and is not far removed from transverse division (for instance, the strobilation of the acraspedæ medusæ and the chain tape-worms). Lateral budding is much more common; it determines the branching of trees and generally of complex plants, and also of the tree-shaped stocks of sponges, cnidaria (polyps, corals), bryozoa, etc.

A third form of asexual reproduction is the formation of spores or "germ-cells," which are usually produced in great numbers inside the organism, then detached from it, and developed into new organisms without needing fertilization. The spores are sometimes motionless (rest-spores or paulospores); sometimes they have one or more lashes which enable them to swim about (rambling-spores or planospores). This monogenetic propagation is very common among the protists, both protophyta and protozoa. Among the latter the sporozoa (gregarinæ, coccidia, etc.) are remarkable for the passing away of the whole unicellular organism in the formation of spores; in this case and in many of the rhizopods (_mycetozoa_) the process coincides with manifold cell-division. In other cases (radiolaria, thalamophora) only a portion of the parental cells is used for the production of spores. Spore-formation is very common among the cryptogams; here it usually alternates with sexual propagation. The spores are generally formed in special spore-capsules (sporangia). In the flowering plants (anthophyta) sporogony has disappeared. It is found at times in the tissue-animals (in the fresh-water sponges); in this case the sporangia are called _gemmulæ_.

The essential feature of sexual generation is the coalescence of two different cells, a female ovum (egg-cell) and a male sperm-cell. The simple new cell which arises from the blending of these is the stem-cell (_cytula_), the stem-mother of all the cells that make up the tissues of the histon. But even among the unicellular protists we find in many places the beginnings of sexual differentiation; it is foreshadowed in the blending or copulation of two homogeneous cells, the gameta. We may conceive this process, or zygosis, as a peculiar and very favorable kind of growth, that is connected with a rejuvenescence of the plasm; the latter is enabled to propagate by repeated cleavage through the mixing of the two different plasma-bodies on either side (_amphimixis_). When these two gameta become unequal and differ in size and shape, the larger female body is called the macrogameton or macrogonidion, and the smaller, male part, the microgameton or microgonidion. Among the histona the first is called the egg-cell (_ovulum_), and the latter the sperm-cell (_spermium_, or _spermatozoon_). As a rule the latter is a very mobile ciliated cell, the former an inert or amœboid cell. The vibratory movements of the sperm-cells serve for approaching the ovulum in order to fertilize it.

The qualitative difference between the two copulating sexual cells (_gonocyta_), or the chemical difference between the ovoplasm of the female and the sperm-plasm of the male cell, is the first (and often the only) condition of amphigony; subsequently we find in addition (in the higher histona) a very elaborate apparatus of secondary structures. With this chemical difference is associated a peculiar double form of sensitive perception and an attraction based thereon, which is called sexual chemotaxis or erotic chemotropism. This "sex-sense" of the two gonocyta, or elective affinity of the male androplasm and the female gynoplasm, is the cause of mutual attraction and union. It is very probable that this sexual sense-function, akin to smell or taste, and the movements it stimulates, are located in the cytoplasm of the two sex-cells, while heredity is the function of the caryoplasm of the nucleus. (_Cf._ the _Anthropogeny_, chapters vi. and vii.)

The sexual difference between the two forms of gonoplasm, the ovoplasm of the female and spermoplasm of the male cell, is noticeable at the very beginning of sexual differentiation in the different sizes of the copulating gameta, and later in their increasing divergence as to shape, composition, movement, etc. It leads further to the distribution of the germinal regions (in which the sex-cells are formed) into two different individuals. When the ovum and the sperm-cell are produced in one and the same individual, we call this an hermaphrodite; and when they are formed in two different individuals (male and female), we call them monosexual, or gonochorists. In accordance with the various stages of individuality which we distinguished above (chapter vii.), we may indicate the following stages of hermaphrodism and gonochorism.

Some groups of protists, especially the highly organized ciliated infusoria (_ciliata_), are distinguished by having a separation of male and female plasm within the unicellular organism. The ciliata propagate, as a rule, in large numbers by repeated division (by indirect cell-cleavage). But this monogony has its limits, and has to be interrupted from time to time by amphigony, a rejuvenation of the plasm, which is effected by the conjugation of two different cells and the partial destruction of their nuclear matter. By conjugation is meant the partial and momentary union of two different unicellulars, while copulation is a total and permanent coalescence. When two ciliated infusoria conjugate they place themselves side by side, and connect for a time by means of a bridge of plasm. A part of the nucleus of each has already divided into two portions, one of which functions as the female standing-nucleus (_paulocaryon_) and the other as the male travelling-nucleus (_planocaryon_). The two mobile nuclei enter the plasm-bridge, and move through it, pushing against each other, into the body of the opposite cell; they then coalesce with the deeper lying standing-nucleus. When a fresh nucleus has been thus formed (by _amphimixis_) in each of the copulating cells, they again separate. The two rejuvenated cells have once more acquired the power to propagate for a long time by division.

This peculiar hermaphroditic formation of the cells, which distinguishes the ciliated infusoria and some other protists, and which we now know in its smallest details through the investigations of Richard Hertwig, Maupas, and others, is especially interesting because it proves that the chemical difference between the female gynoplasm and the male androplasm can be found within a single cell. This erotic division of labor is so important that formerly it was universally ascribed to two different cells. Recent accurate research, penetrating into the smallest visible processes of fertilization, has shown that the essential feature in the formation of a fresh individual (the stem-cell) is the blending of equal portions (hereditary parts) of the male and female nuclei; the caryoplasm of the two copulating cells is the vehicle of heredity from the parents. The cytoplasm of the cell-body, on the other hand, serves the purposes of adaptation and nutrition. As a rule the cell-body of the ovulum is very large, and is, as a food-store, very richly provided with albumin, fat, and other nutritive matter (food-yolk); while the cytoplasm of the sperm-cell is very small, and generally forms a vibrating lash, with which it moves along and seeks the ovum.

In most of the plants the female and male cells are produced by the same sprout, and in many of the lower animals by one and the same person. This kind of hermaphrodism in "individuals of the second order" is called monoclinism ("one-beddedness"). In many of the higher plants (monœcic stocks) and most of the higher animals we have diclinism ("two-beddedness")--in other words, the one sprout or person has only male, and the other sprout or person only female, organs--this is gonochorism of individuals of the second order. Monoclinism is generally associated with sedentary life (and often necessary for it), and diclinism with free movement. Adaptation to parasitic habits also favors monoclinism; thus, the crabs, for instance, are for the most part gonochoristic individuals, but the creeping crabs (_cirripedia_), which have adopted sedentary (and to an extent parasitic) habits, have become hermaphrodites in consequence. Many intestinal parasites among the lower animals (such as tape-worms, suctorial worms, wonder-snails), which live isolated lives inside other animals, have to be hermaphroditic and able to fertilise themselves if the species is to be maintained. On the other hand, many hermaphroditic flowers, although they have both sorts of sex-organs, are incapable of fertilizing themselves and have to receive this from insect visitors which carry the pollen from one flower to another.

Individuals of the third order, which we call stocks (_cormi_) in both the plant and animal worlds, also exhibit varying features in the sex-persons which compose them. When male and female diclinic sprouts or persons are found side by side on the same stock, we call this hermaphrodism of the cormi _monœcia_ ("one-housedness"); this is the case with most of the siphonophora and some of the corals. _Diœcia_ ("two-housedness") is less common: in this one stock has only male and the other only female sprouts or persons, as in poplars and osiers, most of the corals, and some of the siphonophora. The physiological advantages of crossing--the union of sex-cells of different individuals--favor progressive sex-division in the higher organisms.

A comparative study of the features of hermaphrodism and sex-division in the plant and animal worlds teaches us that both forms of sex-activity are often found in closely related organisms of one and the same group, sometimes even in different individuals of the same species. Thus, for instance, the oyster is usually gonochoristic, but sometimes hermaphroditic; and so with many other mollusks, vermalia, and articulata. Hence, the question often raised, which of the two forms of sex-division is original, is hardly susceptible of a general answer, or without relation to the stage of individuality and the place in classification of the group under discussion. It is certain that in many cases hermaphrodism represents the original feature; for instance, in most of the lower plants and many of the stationary animals (sponges, polyps, platodes, tunicates, etc.). Where we find exceptions in these groups, they are of secondary origin. It is equally certain, on the other hand, that in other cases the separation of the sexes is the primitive arrangement; as in siphonophoræ, ctenophoræ, bryozoa, cirripedia, and mollusks. In these cases the hermaphrodism is clearly secondary in the sense that the hermaphrodites descend originally from gonochorists.

It is only in a few sections of the lowest histona that the two kinds of sex-cells arise without a definite location in different parts of the simple tissue, as in a few groups of the lower algæ and in the sponges. As a rule they are formed only at definite positions and in a special layer of the tissue-body, and mostly in groups, in the shape of sexual glands (_gonades_). These bear special names in different groups of the histona. The female glands are called archegonia in the cryptogams, _nucellus_ (formed from the macrosporangia of the pteridophyta) in the phanerogams, and ovaries in the metazoa. The male glands are called antheridia in the cryptogams, pollen-sacs (formed from the microsporangia of the ferns) in the phanerogams, and testicles (_as spermaria_) in the metazoa. In many cases, especially in aquatic lower animals, the ovula (as products of the ovaries) are discharged directly outward. But, in most of the higher organisms, special sexual ducts (_gonoductus_) have been formed to conduct both kinds of the gonocyta out of the organism.

While the two kinds of sexual glands are usually located in different parts of the generating organism, there are, nevertheless, a few cases in which the sex-cells are formed directly and together from one and the same gland. These glands are called hermaphroditic glands. Such structures are very notable in several highly differentiated groups of the metazoa, and have clearly been developed from gonochoristic structures in lower forms. The class of crested medusæ, or ribbed medusæ (ctenophoræ), contains glasslike, sea-dwelling cnidaria of a peculiar and complicated build, which probably descend from hydromedusæ (or craspedota). But whereas the latter have very simple gonochoristic structures (four or eight monosexual glands in the course of the radial canals or in the gastric wall), in the ctenophoræ the eight hermaphroditic canals run in a meridian arch from one pole of the cucumber-shaped body to the other. Each canal corresponds to a ciliary streamer, and forms ovaries at one border and testicles at the other; and these are so arranged that the eight intercostal fields (the spaces between the eight streamers) are alternately male and female. Still more curious are the hermaphroditic glands of the highly organized, land-dwelling, and air-breathing lung-snails (_pulmonata_), to which our common garden snail (_arion_) and vineyard snail (_helix_) belong. Here we have a hermaphroditic gland with a number of tubes, each of which forms ovaries in its outer part and sperma in the inner. Still the two kinds of sex-cells lead separately outward.

In most of the lower and aquatic histona both kinds of sex-cells, when they are ripe, fall directly into the water, and come together there. But in most of the higher, and especially the terrestrial, organisms special exits or conducting canals have been formed for the sex-products, the sexual ducts (_gonoductus_); in the metazoa the female have the general name of oviducts and the male spermaducts (or _vasa deferentia_). In the viviparous histona special canals serve for the conveyance of the sperm to the ovum, which remains inside the mother's body; such are the neck of the archegonium in the cryptogams, the pistil in the phanerogams, and the vagina in the metazoa. At the outer opening of these conducting canals special copulative organs are developed, as a rule.

When the ejected sex-cells do not directly encounter each other (as in many aquatic organisms), special structures have to be formed to convey the fertilizing sperm from the male to the female body. This process of copulation becomes important, as it is associated with characteristic feelings of pleasure, which may cause extreme psychic excitement; as sexual love it becomes, in man and the higher animals, one of the most powerful springs of vital activity. In many of the higher animals (namely, vertebrates, articulates, and mollusks) there are also formed a number of glands and other auxiliary organs which co-operate in the copulation.

The manifold and intimate relations which exist, in man and the higher animals (especially vertebrates and articulates), between their sexual life and their higher psychic activity, have given rise to plenty of "wonders of life." Wilhelm Bölsche has so ably described them in his famous and popular work, _The Life of Love in Nature_, that I need only refer the reader to it. I will only mention the great significance of what are called "secondary sexual characters." These characteristics of one sex that are wanting in the other, and that are not directly connected with the sexual organs--such as the man's beard, the woman's breasts, the lion's mane, or the goat's horns--have also an æsthetic interest; they have, as Darwin showed, been acquired by sexual selection, as weapons of the male in the struggle for the female, and vice versa. The feeling of beauty plays a great part in this, especially in birds and insects; the beautiful colors and forms which we admire in the male bird of paradise, the humming-bird, the pheasant, the butterfly, etc., have been formed by sexual selection (_cf._ the _History of Creation_).

In various groups of the histona the male sex has become superfluous in the course of time; the ovula develop without the need of fertilization. That is particularly the case in many of the platodes (trematodes) and articulates (crustacea and insects). In the bees we have the remarkable feature that it is only decided at the moment of laying the egg whether it is to be fertilized or not; in the one event a female and in the other a male bee is formed from it. When Siebold proved at Munich these facts of miraculous conception in various insects, he was visited by the Catholic archbishop of the city, who expressed his gratification that there was now a scientific explanation possible of the conception of the Virgin Mary. Siebold had, unfortunately, to point out to him that the inference from the parthenogenesis of the articulate to that of the vertebrate was not valid, and that all mammals, like all other vertebrates, reproduce exclusively from impregnated ova. We also find parthenogenesis among the metaphyta, as in the _chara crinita_ among the algæ, the _antennaria alpina_ and the _alchemilla vulgaris_ among the flowering plants. We are, as yet, ignorant for the most part of the causes of this lapse of fertilization. Some light has been thrown on it, however, by recent chemical experiments (the effect of sugar and other water-absorbing solutions), in which we have succeeded in parthenogenetically developing unfertilized ova.

In the higher animals the complete maturity and development of the specific form are requisite for reproduction, but in many of the lower animals it has been observed recently that ovula and sperm-cells are even formed by the younger specimens in the larva stage. If impregnation takes place under these conditions, larvæ of the same form are born. And when these larvæ have afterwards reached maturity and reproduced in this form, we call the process _dissogony_ ("double-generation"). It is found in many of the cnidaria, especially the medusæ. But if larvæ propagate by unfertilized ova, and so reproduce their kind parthenogenetically, the process is known as _pædogenesis_ ("young-generation"). It is found particularly in the platodes (trematodes) and some of the insects (larvæ of _cecidomyca_ and other flies).

In a large number of lower animals and plants sexual and asexual generation regularly alternate. Among the protists we find this alternation of generation in the sporozoa; among the metaphyta in the mosses and ferns; and among the metazoa in the cnidaria, platodes, tunicates, etc. Often the two generations differ considerably in shape and degree of organization. Thus, in the mosses the asexual generation is the spore-forming moss capsule (_sporogonium_), while the sexual is the moss plant with stalk and leaves (_culmus_). In the case of the ferns, on the other hand, the latter is spore-forming and monogenetic, while the thallus-formed, simple, and small fore-germ (_prothallium_) is sexually differentiated. In most of the cnidaria a small stationary polyp is developed out of the ovum of the free-swimming medusa, and this polyp, in turn, generates by budding medusæ, which reach sexual maturity. In the tunicates (salpa) a sexual social form alternates with an asexual solitary form; the chain-salpa of the former are smaller and differently shaped than the large individual salpa of the latter, which again generate chains by budding. This special form of metagenesis was the first to be observed, as it was in 1819 by the poet Chamisso, when he sailed round the world. In other cases (for instance, in the closely related _doliolum_) a sexual generation alternates with two (or more) neutral ones. The explanation of these various forms of alternating generations is given in the laws of latent heredity (atavism), division of labor, and metamorphosis, and especially by the biogenetic law.

While in real metagenesis (alternation of generations in the strict sense) the asexual generation propagates by budding or spore-formation, this is done parthenogenetically in the cognate process of heterogenesis. This it is which, especially in many of the articulates, causes an immense increase of the species in a short time. Among the insects we have the leaf-lice (aphides), and among the crustacea the water-fleas (daphnides), that propagate in great numbers during warm weather by unfertilized "summer-ova." It is not until the autumn that males appear and fertilize the large "winter-ova"; in the following spring the first parthenogenetic generation issues from the winter eggs. The two heterogenetic generations are very different in the parasitic suctorial worms (trematodes). From the fertilized ovum of the hermaphrodite distoma we get simply constructed nurses (pædogenetic larvæ), inside which cercaria are generated from unfertilized ova; these travel, and are afterwards converted (inside another animal) into distoma once more.

I have given (_General Morphology_, chap, ii., p. 104) the name of strophogenesis to the complicated process of cell-reproduction, which we find in the ontogeny of most of the higher histona, both phanerogams and cœlomaria. In these there is not a real alternation of generations, as the multicellular tissue-forming organism develops directly from the impregnated ovum. But the process resembles metagenesis in so far as the ontogenetic construction consists itself in a repeated division of the cells. Many generations of cells proceed by cleavage from the one stem-cell (the impregnated ovum) before two of these cells become sexually differentiated, and form a generation of sexual cells. However, the essential difference consists in the fact that all these generations of cells--in the body of both the higher animals and the flowering plants--remain joined together as parts of a single bion (a unified physiological individual); but in the alternation of generations each group produced is made up of a number of bionta, which live as independent forms--often so different from each other that they were formerly thought to be animals of separate classes, such as the polyps and medusæ. Hence we must not describe the reproductive circle of the phanerogams as an alternation of generations, although it has started from the fern (by abbreviated heredity).

All simple forms of sexual reproduction without alternation of generations are comprised under the title of _hypogenesis_. The generative cycle proceeds from ovum to ovum in one and the same bion or physiological individual. This form of development is usual with most of the higher animals and plants; it may proceed with or without metamorphosis. The younger forms which arise temporarily in the latter case, and are distinguished from the sexually ripe form by the possession of the provisional (and subsequently disappearing) organs--larva organs (for instance, the tadpole or the pupa), are comprised under the general head of larvæ.

As a rule, only organisms of the same species seem to have sexual union and generate fertile progeny. This was formerly a rigid dogma, and served the purpose of defining the loose idea of the species. It was said: "When two animals or plants can have fertile offspring they belong to the same real species." This principle, which once afforded support to the dogma of the constancy of species, has long been discarded. We now know by numbers of sound experiments that not only two closely related species, but even two species of different genera, may have sexual intercourse in certain circumstances, and that the hybrids thus generated can have fertile offspring, either by union among themselves or with one of the parents. However, the disposition to hybridism varies considerably, and depends on the unknown laws of sexual affinity. This sexual affinity must be based on the chemical properties of the plasm of the copulating cells, but it seems to show a good deal of vagueness in its effect. As a rule, hybrids exhibit a combination of the features of both parents.

It has been proved by many recent experiments that hybrids have a more powerful build and can reproduce more strongly than pure offspring, whereas pure selection has generally in time an injurious effect. A freshening by the introduction of new blood seems to be good from time to time. Hence, it is just the reverse of what the former dogma of the constancy of species affirmed. The question of hybridism has, generally speaking, no value in defining the species. Probably many so-called "true species," which have relatively constant features, are really only permanent hybrids. This applies especially to lower sea-dwelling animals, the sexual products of which are poured into the water and swarm together in millions. As we know of various species of fishes, crabs, sea-urchins, and vermalia, that their hybrids are very easily produced and maintained by artificial impregnation, there is nothing to prevent us from believing that such hybrids are also maintained in the natural state.

The short survey we have made of the manifold varieties of reproduction is sufficient to give an idea of the extraordinary wealth of this world of wonders. When we go more closely into details we find hundreds of other remarkable variations of the process on which the maintenance of the species depends. But the most important point of all is the fact that all the different forms of tocogony may be regarded as connected links of a chain. The steps of this long ladder extend uninterruptedly from the simple cell-division of the protists to the monogony of the histona, and from this to the complicated amphigony of the higher organisms. In the simplest case, the cell-cleavage of the monera, propagation (by simple transverse division) is clearly nothing more than transgressive growth. But even the preliminary stage of sexual differentiation, the copulation of two equal cells (_gameta_), is really nothing but a special form of growth. Then, when the two gameta become unequal in the division of labor, when the larger inert macrogameton stores up food in itself, and the smaller, mobile microgameton swims in search of it, we have already expressed the difference between the female ovum and the male sperm-cell. And in this we have the most essential feature of sexual reproduction.

The reproduction of the organism is often regarded as a perfect mystery of life, and as the vital function which most strikingly separates the living from the lifeless. The error of this dualistic notion is clear the moment one impartially considers the whole gradation of forms of reproduction, from the simplest cell-division to the most elaborate form of sexual generation, in phylogenetic connection. It is obvious all through that transgressive growth is the starting-point in the formation of new individuals. But the same must be said of the multiplication of inorganic bodies--the cosmic bodies on the larger scale, crystals on the smaller scale. When a rotating sun passes a certain limit of growth by the constant accession of falling meteorites, nebulous rings are detached at its equator by centrifugal force, and form into new planets. Every inorganic crystal, too, has a certain limit of individual growth (determined by its chemical and molecular constitution). However much mother-water you add, this is never passed, but new crystals (daughter-crystals) form on the mother-crystal. In other words, growing crystals propagate.

XII

MOVEMENT

Mechanics as the science of motion (kinematics and phoronomism)--Chemistry of vital movement--Active and passive movements--Undulatory movement--Mechanism of imbibition--Autonomous and reflex movements--Will and willing--Mixed movements--Movements of growth--Direction of the vital movement--Direction of the crystallizing force--Direction of cosmic motion--Movements of protists--Amœboid, myophenous, hydrostatic, secretory, vibratory movements: cilia and lashes--Movements of histona, metaphyta, and metazoa--Locomotion of tissue animals: ciliary motion and muscular movements--Muscles of the skin--Active and passive organs of movement--Radiata, articulata, vertebrata, mammalia--Human movements.

All things in the world are in perpetual motion. The universe is a _perpetuum mobile_. There is no real rest anywhere; it is always only apparent or relative. Heat itself, which constantly changes, is merely motion. In the eternal play of cosmic bodies countless suns and planets rush hither and thither in infinite space. In every chemical composition and decomposition the atoms, or smallest particles of matter, are in motion, and so are the molecules they compose. The incessant metabolism of the living substance is associated with a constant movement of its particles, with the building up and decay of plasma-molecules. But here we must disregard all these elementary kinds of movement, and be content with a brief consideration of those forms of motion which are peculiar to organic life, and a comparison of them with the corresponding motions of inorganic bodies.

The science of motion, or mechanics, is now taken in very different senses: (1) in the widest sense as a philosophy of life [generally called mechanism or mechanicism in England], equivalent to either monism or materialism; (2) in the stricter sense as the physical science of motion, or of the laws of equilibrium and movement in the whole of nature (organic and inorganic); (3) in the narrowest sense as part of physics, or dynamics, the science of moving forces (in opposition to statics, the science of equilibrium); (4) in the purely mathematical sense as a part of geometry, for the mathematical definition of magnitudes of movement; and (5) in the biological sense as phoronomy, the science of the movements of organisms in space. However, these definitions are not yet universally adopted, and there is a good deal of confusion. It would be best to follow the lead of Johannes Müller, as we are going to do here, and restrict the name phoronomy to the science of the vital movements which are peculiar to organisms, in contrast to kinematics, the exact science of the inorganic movements of all bodies. The real material object of phoronomy is the plasm, the living matter that forms the material substratum of all active vital movements.