The History of Biological Theories

These new ideas of individuality reached their highest development in the theories of Driesch. 1 1 A philosophical analysis of the problem of individuality will be found in H. Driesch's The Science and Philosophy of the Organism, London, 1907-8, 2 vols. DARWIN'S attack on the constancy of species failed. Only his most faithful disciples Haeckel, Schleiden, Schmidt, and Carpenter accepted the suggestion that there are no species ; Haeckel and Carpenter were the only workers who tried to follow up this idea practically, and in this they had no lasting success. The other Darwinists hold in theory that the word species does not correspond to any existing reality, but in practice they go on discover- ing new species, as was done in pre-Darwinian days. Buffon, attacking Linnaeus, said that there are no such things as 'natural' species or 'natural' families, nor any higher 'natural' groups ; yet he treated of birds in one part of his book, and of mammals in another; he described horses in one article, donkeys in another! The Darwinists ask us to consider the facts of astronomy ; it is probable that no star is absolutely still, nevertheless astronomers speak of fixed stars those whose motion can hardly be detected ; in the same manner, they point out, the varia- tion in any species is usually so slight that we can afford to neglect it in practice.

The larger groups orders, families, classes, &c. have received less attention. The earlier belief that each organ- ism exhibits definite characters defining its species, its natural order, and so on, was held to be untenable. If any one asserted the opposite view, as did Cope, his asser- tion passed unnoticed. Attention was concentrated on the nature of a species. Of course, there are species in Nature. Any one who has once seen a domestic mouse, an apple, a man, would recognize these again. What, then, is the nature of a species ? Linnaeus, to whom more than to any other we

owe the modern conception of a species, assumed that it consisted of an assemblage of external characters, such, for example, as size, colour, form, number of teeth, of stamens, and so on. His followers immersed themselves in the dull recounting of the specific characters of certain organisms. Cuvier's idea of a species was more comprehensive. He regarded it as a group of anatomical characters which, taken together, gave us a fully defined unit. The triangle is not only a collection of sides and angles, it represents a definite unified whole. So, according to Cuvier, those features which characterize a definite group of animals, are in no wise 'disjecta membra' ; combined, they blend harmoniously to form a single unit. Cuvier was an anato- mist, and focussed his attention upon anatomical charac- ters, though he did not deny that to define a species fully certain physiological ones were also necessary. Darwin and his school attacked Cuvier's idea of a species, but they, too, devoted their attention to anatomical characters, and returned to the ideas of Linnaeus, with whom they agreed that the species is represented by the sum of its characters.

And yet a species is not fully described by enumerating its specific characters, nor is it sufficient merely to note anatomical characters. Habits of life tend to be constant, not only through a species, but through wider groups. Such names as predatory animals, birds of prey, hyaenas, nightingales, owls, do not merely suggest animals of such- and-such bodily characters ; they remind us that predatory animals, for example, are flesh-eaters (though the cats, the dog-like animals, the hyaenas, hunt their prey in a diverse manner) ; owls are night-flying birds ; nightingales are good singers ; ants are not merely characterized by the veins in their wings, and by the protuberances on each segment of the body ; a further characteristic is their very highly specialized social life.

Yes even such an elusive character as the numerical relationship between the sexes is constant, and so becomes a characteristic typical of the species. Further, it is the union in one organism of these characters, not the mere fact that there is a series of them, that defines the species. The species lion, for example, expresses an ideal unity, although it is an actual existent (just as a quadrilateral, or gravitation, are abstractions, although real). It is a unity composed of physiological, embryological, psycho- logical, and other characters. This does not exhaust the peculiarity of this idea of a species. One characteristic of the conception and this is true of the whole world of living organisms is that the species cannot be fully realized from one individual. A male and a female, a caterpillar and a butterfly, several strains, and so on these constitute species.

Philosophers have sought in vain for any other pheno- menon, either in the living or in the non-living world, comparable to the relationship between the male and the female. Each sex seems to be, in body, in function, in soul, a finished whole, and yet only together do they form a really complete unit. The following degrees of sexual differentiation are to be distinguished : (a) Asexual organisms; to this group belong certain fungi, all bacteria, and some algae.

(b) Organisms in which the sexual differentiation is only physiological. Two organisms which are apparently abso- lutely identical unite for reproduction. Good examples are provided by the protozoa. In other cases the differ- ences between the male and female cells are sometimes visible to the eye, as in some algae. By far the most numerous are organisms with a definite sexual reproduction. These are either hermaphrodites, or they consist of two separate sexes.

(c) Hermaphrodites are often so constructed that they can be either self-fertilized or cross-fertilized. The cleisto- gamous flowers of many plants are always self-fertilized. These are small flowers which never open, and which occur in addition to the ordinary flowers. (d) Some hermaphrodites contain the organs of both sexes, and yet two individuals unite for fertilization. This is the case in most of the phanerogamic flowers, where the pollen of one blossom is carried to the pistil of another. In hermaphrodite animals cross-fertilization is apparently the rule; self-fertilization only occurs when a cross is impossible.

(e) There are some hermaphrodites which possess the organs of both sexes, and yet are so highly differentiated that the male organs of one individual only fit the female organs of some of their kind, and vice versa. In the primroses, for example, there are two types of flower ; in the one (called 'thrum-eyes') the stamens are above the stigma ; in the other ('pin-eyed') the positions are reversed. Fertilization leads to the best results, when pollen from the shorter stamens is deposited on the shorter pistil, and that from the longer stamens on the longer pistil. Other arrangements occur which necessitate cross-fertilization even among hermaphrodites; for example, one of the sexual products (either the male or the female) may be ripe before the other. In the Lime the stamens ripen in every flower before the pistil ; in Aristolochia the reverse is the case. Analogous phenomena are seen among the Molluscs and the Tunicates.

(/) Darwin discovered that some hermaphrodite Cirri- pedes (small fixed Crustacea) produce, in addition to the normal form, small parasitic males ; a similar relationship obtains among the Myzostomata, a peculiar group of worms. Many plants bear, in addition to perfect flowers, others which only have stamens (e.g. Gallium cruciatum) or only pistils (e.g. Or ache). (g) Finally, there are the organisms in which the sexes are quite distinct. They complete each other, not only in the structure and function of their sex organs, but also in temperament. The male is active, aggressive, mobile, and seeks the more passive female. This fundamental difference

may go further, and male and female may differ in the so-called secondary sexual characters also that is, in characters other than those serving the actual act of fertilization. As examples of these we may cite the beard, the greater hairiness and strength of the man, and many similar features. Males tend to have more perfect organs of sense and of locomotion, to have more brightly coloured feathers, to possess antlers and other adornments. They are better singers, emit peculiar odours animal beauty, in fact, often depends on the special development of these secondary characters. The flowers of dioecious and of monoecious plants are often distinguishable by characters other than those of the reproductive organs. In Catasetum, among the orchids, the flowers which look like a Medusa's head are so different that they were thought to belong to three different species : at one time the male was called Catasetum, the female Monachanthus, the hermaphrodite flower Myanthus.

Sex, then, usually expresses itself in the existence of two individuals, often very different in their appearance and in their mode of life ; but when we have stated this fact we have by no means exhausted the phenomena of sex. (b) Among the social insects bees, ants, and termites only a few individuals are sexually developed. Others the worker bees, for example possess degenerate repro- ductive, organs and are different in appearance from the perfect individuals. Sometimes three or more different types of individual exist. Among bees there are three types. Forel (1904) describes ten different types in certain ant societies, and all these forms belong to one species.

(*) Among certain insects the one sex has one form only, the other two different forms. There is one form of the male butterfly Papilio merope but several types of female. Some species of wasp (certain of the Chalcididae) have one type of female, but two different kinds of male. (J) Finally, we must point out the remarkable fact that, for each species, the numerical ratio of males to females is a constant one. The nature of sex has not often been the subject of phylogenetic discussion. Darwin himself directed his attention entirely to the secondary sexual characters, and Darwinian speculations concerning origins were concerned only with these.

In connexion with the question of the meaning of sex, it is often asked whether the male or the female is more highly developed. As is well known, Aristotle asserted that the female is a lower type than the male. Darwin (Descent of Man) also thought that, as a rule, the female is the less developed, and many share this view with him. Von Kennel asserted that the female represents a degene- rate type the ovaries need much nourishment, and so the growth of other organs is adversely affected. Only a few workers have held the opposite view. Montgomery was among these; he supported his ideas by the following arguments :

(i) Males are more often completely degenerate than are females, e.g. the males of Rotifera, of the worm Bonellia, of the Cirripedes. Females, as a rule, only have some organs abortive, e. g. the females of some insects have abortive wings. (ii) Hermaphrodites are usually protandrous, i.e. the sperms mature earlier and hence at a lower stage of development than the eggs. (iii) In most invertebrate animals the females are longer- lived, are larger, have more complicated sexual organs, and are entrusted with the care of the new generation. This must be regarded as a higher state of development, in spite of the fact that the male has more perfect organs of sense for locomotion and copulation. Among vertebrates the males have, as a rule, a simpler urino-genital duct, the females having separate ducts. In comparison with these facts the secondary sexual characters are unimportant, for, according to most phylogenetic theories, they are of more recent date.

Most naturalists, then, have held that the male is the more highly developed form ; only a few have raised their voices in favour of the opposite view. The supporters of feminine emancipation, however, have turned even to biology to find support for their assertion that the female is intellectually the equal of the male. Man is first an egg-cell, then embryo, infant, youth, man, and old man, and the name 'human being' does not apply to one of these stages only, it applies to all. These manifold forms, developing in course of time, are charac- teristic of the living organism. It is true that water can be now hot, now cold, now in drops, now as snow, ice, vapour but there is no unchanging sequence; it may alter its state, but is not bound to do so; further, the alteration may occur in two directions. An animal can neither remain at the same stage of development, nor can it go backwards. An organism, then, develops as a series of forms following one another in time the one passing into the other. Sometimes certain stages in this series are so complete in themselves, that they form a rounded whole ; the whole life is then like a chain, consisting of a series of links, each one obviously more or less complete in itself.

Even among human beings this is just suggested, for the embryo 'in utero' leads a life very sharply marked off from its independent life as an infant. The following cases of metamorphosis during development are the ones usually quoted: (a) Development without metamorphosis; among the lizards the embryo develops within the egg, and when it escapes it resembles the adult, although it is small. (&) Development with metamorphosis, as is seen in the frog. The embryo becomes a tadpole, which lives in water ; this then changes into the frog, which differs from the tadpole both in structure and in its way of life.

(c) When there is alternation of generations two phases of the life history alternate j these do not pass into one another by metamorphosis; each phase is borne by the other, and begins as a single cell. This mode of life is well seen in mosses and in the higher cryptogams. In the fern, for example, the asexual fern lives and reproduces as an organism complete in itself. After a certain period, or under certain external conditions, it forms spores ; from one of these there develops the sexual plant, which is quite different in appearance from its immediate predecessor. This forms eggs and sperms, and after fertilization, asexual plants develop from the eggs.

There is a certain resemblance between this mode of life and that seen among the infusorians. These reproduce for many generations asexually by fission, but eventually a sexual copulation between two individuals is necessary to stimulate to renewed activity. Among the lower plants sexual and asexual generations may alternate according to varying laws. (d) Heterogony is the name given to that type of alter- nation in which each generation is sexual. The parasitic worm which lives in the lungs of the frog (Rhabdonema) gives birth after fertilization to larvae, which in their turn come to possess fully developed sexual reproductive organs ; from these the original form is once more produced. Heterogony is often seen in parasitic worms.

(e) A sexual generation may alternate with a partheno- genetic one, (by the latter term we indicate embryos that develop from unfertilized eggs). In summer the Aphis only produces females and these reproduce parthenogeneti- cally. In autumn males arise as well; they fertilize the late females, who then lay eggs from which, in spring, a new generation of females arises, to continue the story in that year. In this case the parthenogenetic generations are more numerous than the sexual ones ; sometimes the sexual form is very rare and can even be completely sup- pressed (latent). Some forms of Aphis, some wasps be- longing to the Cynipidae, some Rotifers, certain small Crustacea belonging to the orders Cladocera and Ostra- coda, only reproduce parthenogenetically. Among plants

there are certain ferns in which the sexual generation is in abeyance they reproduce by means of spores, which begin to grow apogamously while still on the leaves. Before the time of Linnaeus, Leibniz had discussed the differences between the individuals of a species. He pointed out that no two things in the world are absolutely identical, since then it would be impossible to distinguish between them. This view divided his followers into two camps ; the one group, the more practically-minded, were convinced from their every-day experience that there are natural groups of organisms (Tournefort stressed species, Linnaeus orders) ; for them Leibnitz's formula meant that every order, every species, passes over into many others. Another group, including Lamarck,preferred Bonnet's more abstract teaching, and interpreted this idea differently. They believed that all organic beings can be arranged in a single series, in which each individual represents a transi- tion between the two others standing next to it in the series, and only between these.

While morphological ideas prevailed, the first of these views was the more generally accepted one. In the early nineteenth century the Swiss morphologist Pyrame De Candolle turned his attention to the question of species. He began by considering a fact already known to Linnaeus, that a chance departure from type may prevent us from recognizing a member of a known species, and that such chance departures may be due to the influence of the environment.

To eliminate the influence of such chance irregularities upon our definition of the species, he called for experi- ments to determine the nature of each species. To describe one or a few individuals is not enough it is im- possible to say which characters are constant, and which fluctuating; each type must be under observation for several successive generations and while exposed to very different conditions. Then, in defining the species, we must only take into consideration characters which remain constant under these conditions. These ideas of De Can- dolle represented a great advance upon merely descriptive methods of defining a species. But the whole discussion was soon turned into new channels.

In the nineteenth century mere classification still flourished in England. Darwin knew little of continental morphology, and his work obviously followed the pre- vailing English ideas. From the discussions of scientists whose somewhat stale ideas were based chiefly on her- barium material, he inferred that individual and chance variations often represent the beginnings of new species. Thus he turned men's minds back to the ideas of Bonnet, Buffon, and Lamarck.

In discussing variability Darwin distinguished between individual differences and more strongly marked variations ; he called the latter c single variations', as when a three-toed horse is born. At first Darwin did not say which type of variation he considered the more important for the origin of species. Later it was pointed out that the strongly marked 'single variations' are of very rare occurrence, and that they must necessarily disappear very soon, since they mate with the typical form. Darwin then admitted that they must be less important than are individual fluctua- tions, as starting-points of new species.

In the early discussions which ranged round the idea of evolution Darwin's reasoning and deductions were merely repeated ; when the first enthusiasm had waned workers turned their attention to his assumptions, and endeavoured to check them and to widen them ; and, as this work pro- gressed, interest in his theories diminished. Observations made by Nageli, Kerner, and Hoffmann now attracted attention ; their views on variation were rather different from those of Darwin. They believed that variations due to the action of the environment, and non-inheritable, are fundamentally different from those inheritable varia- tions which make their appearance as the result of the action of internal causes.

Weismann, in his theoretical discussions, drew a similar distinction between the two types of variation. There was, too, a revival of interest in the views of De Candolle. In France, positivism was flourishing; this philosophy suited the analytical gifts of the French intellect, and served to unite the views of Cuvier and of Lamarck. It led to the abandonment of any endeavour to settle these questions by experimental methods. Just at the time when Darwin was studying domestic varieties, endeavouring to prove that they resemble natural varieties, and may be regarded as the beginnings of new species, the problem of the relation between races and species was being once more discussed in France too.

Quatrefages treated the question from the anthropo- logical point of view, Godron from the zoological, Naudin and Jordan from the point of view of botany. All these workers assumed that species actually exist, and their research was directed to the finding of an exact method of defining the same. Jordan, a botanist, collected species of Draba verna a common weed of dry meadows from all parts of Europe and cultivated its varieties for several successive genera- tions, as De Candolle had advised. He found that this single species includes over two hundred constant and unchanging varieties ; he declared that all these must be regarded as minor species within the larger one. He held that these constant varieties must be distinguished from those artificial ones which revert to the ancestral form in a few generations if left to themselves, as do, for example, the varieties among cultivated fruit trees (1873).

Godron held similar views. His book On Species and Races appeared in the same year as Darwin's great work ; the views there expressed are opposed to those of Darwin. He affirms that wild races, living in a state of nature, are quite constant and sharply distinguished from one another ; variations only arise occasionally, and they are not maintained. Domesticated races form a contrast to these ; they have been bred under artificial conditions, and

have undergone modifications which are transmitted by heredity. Naudin (1865) seems to favour the opposite view. He considers that there is no essential difference between a species, a family, and a variety. Experiments led him to consider that a species is *a group of individuals which resemble one another, and differ from all other groups in greater or less degree, and which retain the same physiognomy and organization through many successive generations'.