thenceforward assured. The irreconcilable feud between the
two leaders really involved a reconciliation for their followers;
theories of homological anatomy had thenceforward to be
strictly subjected to anatomical and embryological verification,
while anatomy and embryology acquired a homological aim.
This union of the solid matter and rigorous method of Cuvier
with the generalizing spirit and philosophic aims of Geoffroy
is well illustrated in the works of Owen.
The further evolution of the idea of homology is sketched below, while the extent and rapidity of the subsequent progress of the knowledge of all the structural aspects of plants and animals alike make a historical survey impossible up to the appearance of the Origin of Species (1859). The needful solution was effected by Darwin. The “Urpflanze” of Goethe, the types of Cuvier, and the like, at once became intelligible as schematic representations of ancestral organisms, which in various and varying environments, have undergone differentiation into the vast multitude of existing forms. All the enigmas of structure become resolved; “representative” and “aberrant,” “progressive” and “degraded,” “synthetic” and “isolated,” “persistent” and “prophetic” types no longer baffle comprehension; conformity to type represented by differentiated or rudimentary organs in one organism is no longer contradicted by their entire disappearance in its near allies, while systematist and morphologist become related simply as specialist and generalizer, all through this escape from the Linnaean dogma of the fixity of species. The phenomena of individual development receive interpretation in terms of ancestral history; and embryology thus becomes divided into ontogeny and phylogeny—the latter, too, coming into intimate relation with palaeontology—while classification seeks henceforth the reconstruction of the genealogical tree. All these results were clearly developed in Haeckel’s Generelle Morphologie (1866), while the valuable contemporaneous Principles of Biology of Herbert Spencer also gave special attention to the relation of morphology to physiology.
Individuality.—Probably no subject in the whole range of biology has been more extensively discussed than that of the nature of organic individuality. The history of the controversy is of interest, since besides leading up to solid results it serves, perhaps better than any other case, to illustrate the slow emergence of the natural sciences from the influence of scholastic thought. Starting from the obvious unity and indivisibleness of Man and other higher animals, and adopting some definition such as that of C. F. B. Mirbel, “Tout être organisé, complet dans ses parties, distinct et séparé des autres êtres, est un individu,” it was attempted times without number to discover the same conception elsewhere in nature, or rather to impose it upon all other beings, plants and animals alike. The results of different inquirers were of course utterly discrepant. It seemed easy and natural to identify a tree or herb corresponding to the individual animal, yet difficulties at once arose. Many apparently distinct plants may arise from a common root, or a single plant may be decomposed into branches, twigs, shoots, buds or even leaves, all often capable of separate existence. These, again, are decomposable into tissues and cells, the cells into nucleus, &c., and ultimately into protoplasmic molecules, these finally into atoms—the inquiry thus passing outside organic nature altogether and meeting the old dispute as to the ultimate divisibility of matter. In short, as Haeckel remarks, scarcely any part of the plant can be named which has not been taken by some one for the individual. It is necessary, therefore, briefly to notice some of the principal works on the subject, and these may conveniently be taken in descending order.
While H. Cassini practically agreed with Mirbel in attempting to regard separate plants as individuals, the widest interpretation of the individual is that of G. Gallesio (1816), who proposed to regard as an individual the entire product of a single seed, alike whether this developed into a uni-axial plant extended continuously like a banyan, or multiplied asexually by natural or artificial means like the weeping-willow or the Canadian pondweed, of each of which, on this view, there is only a single individual in Britain, happily discontinuous.
At once the oldest and most frequently maintained view is that which regards the bud or shoot consisting of a single axis with appendages as the plant-individual, of which the tree represents a colony, like a branched hydroid polyp. This conception, often attributed to Aristotle, but apparently without foundation, appears distinctly in the writings of Hippocrates and Theophrastus—the latter saying, “The bud grows on the tree like a plant in the ground.” The aphorism of Linnaeus, “Gemmae totidem herbae,” is well known; and in this view C. F. Wolff and Humboldt concurred, while Erasmus Darwin supported it by an appeal to the facts of anatomy and development. The most influential advocate of the bud theory during the first half of the 19th century was, however, Du Petit-Thouars, who, although starting much as usual with a “principe unique d’existence,” supported his theory on extensive though largely incorrect observations on stem structure and growth. For him the tree is a colony of phytons, each being a bud with its axillant leaf and fraction of the stem and root. Passing over numerous minor authors, we come to the central work of Alex. Braun (1853), in which, as Sachs has clearly pointed out, the illegitimate combination of Naturphilosophie with inductive morphology reaches its extreme. He reviews, however, all preceding theories, admits the difficulty of fixing upon any as final, since the plant, physiologically considered, is rather a dividuum than an individuum, and proposes as a compromise, or indeed as a partial cutting of the knot, the adoption of the shoot, as the morphological individual, comparable to an animal, especially because, unlike the cell, leaf, &c., it includes all the representative characters of the species. Darwin and Spencer on the whole also accept the bud or shoot as at any rate the most definite individual.
The theory of metamorphosis naturally led Goethe, Oken and others to regard the leaf as the individual, while Johannes Muller, J. J. S. Steenstrup and others adopted the same view on various physiological grounds. C. Gaudichaud elaborated a theory intermediate between this view and that of Du Petit-Thouars, according to which the plant was built up of individuals, each consisting of a leaf with its subjacent internode of stem, which was regarded as the leaf-base, and this was supported by Edward Forbes and others, while the nominally converse view—that of the leaf as a mere outward expansion of the stem-segment—was proposed by C. F. Hochstetter.
Though sundry attempts at identifying various tissues, such as the fibro-vascular bundles, as the constituent individuals may be passed over, those associated with the cell theory are of great importance. T. Schwann decided in favour of the cell and regarded the plant as a cell-community, in which the separate elements were like the bees of a swarm—a view virtually concurred in in all essential respects by M. Schleiden, R. Virchow and other founders of the cell theory. Yet, although the structure and functions of the plant are ultimately and specially cellular, it is impossible to ignore the fact that, save in the very lowest organisms, these are subordinated and differentiated into larger aggregates, and form virtually but the bricks of a building, and hence the later theories outlined above. Of attempts to find the individual in the nucleus or the protoplasm granules it is unnecessary to speak further.
So far the theories of absolute individuality. The conception of relative individuality was first clearly expressed by Alphonse de Candolle and Schleiden, both of whom take the cell, the shoot and the multi-axial plant as forming three successive and subordinated categories. K. W. von Nägeli too recognized not only the necessity of establishing such a series (cell, organ, bud, leafy axis, multi-axial plant) but the distinction between morphological and physiological individualities afterwards enunciated by Haeckel.
Passing over the difficulties which arise even among the Protozoa we find that a similar controversy (fully chronicled in Haeckel’s Kalkschwämme) has raged over the individuality of sponges. While the older observers were content to regard each sponge-mass as an individual, a view in which J. N. Lieberkuhn and other monographers substantially concurred, the application of the microscope led to the view suggested by James Clark, and stoutly supported by Saville Kent, that the sponge is a city of amoeboid or infusorian individuals. H. J. Carter looked upon the separate ampullaceous sacs as the true individuals, while others, defining the individual by the possession of a single exhalent aperture, distinguish sponges into solitary and social.
For the higher animals the problem, though perhaps really even more difficult, is less prominent. As Haeckel points out, the earlier discussions and even the comparatively late essay of Johannes Muller take an almost purely psychological or at least a physiological point of view; and the morphological aspect of the inquiry only came forward when the study of much lower forms, such as Cestoid worms (see Platyelmia) or Siphonophores (see Hydrozoa), had raised the difficulties with which botanists had so long been familiar. With the rapid progress of embryology, too, arose new problems; and in 1842 Streenstrup introduced the conception of an “alternation of generations” as a mode of origin of distinct individuals by two methods, for him fundamentally similar, the sexual from impregnated females and the asexual from unimpregnated “nurses”—a view adopted by Edward Forbes and many other naturalists, but keenly criticized by W. B. Carpenter and T. H. Huxley. In R. Leuckart’s remarkable essay on polymorphism (1853) the Siphonophora were analysed into colonies, and their varied organs shown to be morphologically equivalent, while the alternate generations of Steenstrup were reduced to a case of polymorphism in development. Leuckart further partly distinguished individuals of different orders, as well as between morphological and physiological individuals.
In 1852 Huxley, starting from such an undoubted homology as that of the egg-producing process of Hydra with a free-swimming Medusoid, pointed out that the title of individual, if applied to the latter, must logically be due to the former also, and avoided this
confusion between organ and individual by defining the individual
