The History of Biological Theories

Thus the nervous system and the epidermis both originate in the ectoderm, and so were thought to be more nearly related to each other than to bone and muscle, which develop from the mesoderm. The beginning and end portions of the alimentary canal were regarded as differing fundamentally from the middle portion, since they are ectodermal in origin, while the latter is derived from mesoderm. Spiders and insects are nearly related groups, and their excretory organs (the so-called Malpighian tubules) are very similar in appearance. Yet since, in the former group of animals, these develop from the endoderm, while in the latter they arise from the ectoderm, they could not be regarded as homologous structures, in spite of their close morphological and physiological similarity.

This theory of the germinal layers was completed by the theory of the coelom, promulgated by the brothers Oscar and Richard Hertwig. 1 The simplest multicellular animals originate from two embryonic layers ectoderm and endo- derm. But in the course of development there arises, in the embryos of the higher animals, a third layer the mesoderm. This mesoderm arises in some forms, as a pair of diverticula of the endoderm, the so-called coelomic sacs. These sacs represent the beginnings of the body cavity, the so-called coelom. The Hertwigs called a mesoderm of this nature a 'mesoblast'. In other cases no coelomic sacs arise, but ectodermal or endodermal cells (there is no definite rule) pass into the space between these two

1 O. and R. Hertwig, Die Colomtheoric, Versuch einer Erkldrung d. mittleren Keimblattes, Jena, 1881. germinal layers and form there the so-called mesenchyme. In other cases both mesoderm and mesenchyme may be present. The Hertwigs divided all multicellular animals into those which form mesenchyme only and those which form both mesenchyme and a coelom. In addition to this classifica- tion they suggested that the coelom was originally an organ of secretion, and that it afterwards developed the power to form reproductive cells. There was much dis- cussion on the distinction between those animals which possess a coelom and those which do not ; as to which form of coelom is the more primitive that arising as a diverti- culum or the type which is formed by cells of the ecto- derm; also which function is the more primitive the secretory or the production of the sex cells.

Bunge, the physiologist, took all the facts known about the chemical composition of the animal body, and applied to them Haeckel's ' Fundamental Law of Biogenesis'. Common salt is a necessary ingredient of animal food. He tried to explain this by the theory of evolution on the ground of descent. He showed that the amount of salt any body contains depends on the nature of its surround- ings. The only plants rich in sodium chloride are marine plants, sea-shore plants, and the plants inhabiting salt steppes. Chenopodium and Atriplex are exceptions to this rule. These flourish in waste ground rich in salt, and are closely allied to the plant inhabitants of salt steppes. Among invertebrate animals marine creatures and their nearly allied species on the dry land are the only ones with a large salt content. Insects, which are typical land animals, contain extremely little salt. Land-inhabiting vertebrates contain a large amount of salt. This charac- teristic, according to Bunge, is one inherited from marine ancestors. If this be true, Bunge continues, we should expect that the younger any vertebrate animal is the richer its body would be in salt. He asserts that this is the case. The mammalian embryo contains more salt than does the newly born animal, and the

body contains less chlorine and potash as development proceeds. Speculations of this nature reached their culminating point about 1880 and then rapidly declined in importance, as certain abnormal phenomena of development began to attract attention. 1 Haeckel and the earlier embryologists knew of many exceptions to the ordinary course of development, but they were content to call these 'adaptations to special conditions'. They seemed to think that everything that was obscure could be thus explained. All such exceptions were classed together as cases of 'a-typical development' ; among these they distinguished between the following special processes :

(a) Regeneration, by which wounds are healed, and lost parts of the body replaced. (b) Fission, by which process some creatures multiply, e.g. certain worms, which, however, also have a sexual process. The body narrows at one place, and separates into two portions, each of which is capable of becoming a new and complete animal. (c) Budding, by which an animal, such as the freshwater Polyp, forms a new and separate individual from a 'bud' on the parent body.

The study of such phenomena led to the development of a new branch of science experimental morphology, which drove descriptive embryology off the field. It was very difficult to reconcile the phenomena just enumerated with the fundamental assumption of Darwin and Haeckel, that the form of an organism is the result of a series of changes, which have been in progress for millions of years. The fact that an animal can replace a 1 This revolution in the ideas of embryologists is made very clear by a study of the great text-book of comparative embryology published in Jena by E. Korschelt and K. Heider. The first, the part dealing with special embryology, is completely dominated by the theories of Haeckel. It was completed in 1890. The second, the 'general' part, was finished in 1902, and it does not contain any Darwinian discussions, but is concerned with the phenomena of fertilization and with experimental embryology.

lost limb by a new one is as great a blow to Darwinism as is a 'faked antique' to the lover of antiquity. More recently, facts derived from a merely descriptive study of the processes of development have also been cited in dis- proof of Haeckel's fundamental law. Mehnert has suggested that it should be replaced by his principle of 'Kainogenesis'. According to him, homo- logous organs are formed in each developing animal at a rate specific to and quite characteristic of the particular type of animal. The Darwinists asserted that the heart is only a more highly differentiated portion of the great blood-vessels, and it should therefore, according to HaeckeFs law, be formed after these. Nevertheless we know of many vertebrate animals in which it arises before the blood- vessels. Phylogenetically, the mammary glands are sup- posed to be derivatives of the ordinary skin glands, and yet ontogenetically they arise long before the skin glands.

According to Mehnert the rate of ontogenetic develop- ment does not depend upon the phylogeny of the organ, but upon its degree of differentiation. The thigh-bone is, for example, the organ in man which has acquired the greatest ability to grow in length. It develops very quickly in embryonic life. The smallest human member examined by Mehnert was the first phalange of the thumb, and he found that this grows most slowly. 1 Plant embryology was not as much influenced by Haeckel and his views as was zoological theory. From the very beginning von Sachs led botanical speculation into simpler channels. He believed in direct observation rather than in philosophic abstraction, and he regarded development merely from the point of view of the gradual formation of new organs. He distinguished four phases in this process :

(a) The formative period, due to the activity of the apical meristem, or growing point. Here the number and position of the organs are determined, but not their form. 1 E. Mehnert, Biomecbanik, erscblossen aus dem Prinzip der Organogenese, 1898. () The embryonic stage, when those parts of greatest morphological importance are laid down. (c) The elongating stage, when this rudiment is en- larged and assumes its final form. (d) The maturation period, during which the tissues reach their final form. Sachs agreed with Haeckel's funda- mental law of biogenesis in one point : he asserted that the earlier an organ is laid down at the growing point the greater is its phylogenetic importance.

There were other botanists, of whom Strasburger is an outstanding example, who thought that this agreement went much further. EACH race of mankind inhabits its own definite terri- tory, and so do the various species of plants and animals. In one land lives the elephant, in another the gorilla ; the mountain pine and the cedar inhabit different areas. Buffon, in his popular Natural History, gave the impulse which led to the study of many biological problems of this kind. He was the first to ask why the elephant inhabits the warmer regions of the Old World, while one tapir is found in the East Indies and another in South America ; why, in short, the distribution of animals over the surface of the globe does not depend on climate alone. After Buffon, the Russo-German traveller, Pallas, was the worker who added most to our knowledge of animal dis- tribution. His discovery of the rhinoceros and the mammoth, buried under the Siberian ice, attracted wide notice.

Much attention was given to these problems by that versatile German savant, Alexander von Humboldt (1769- 1859), He was a dispassionate investigator; his riches, his public position, and the long voyages he undertook Drought him such fame that to many the whole period will appear as 'the Humboldt period'. Nevertheless, he was essentially unproductive. His genius was not creative ; he hoped to win distinction as a philosopher and a scientist, and yet he had no talent for philosophy, no power to deal with facts in a lively and original way. In addition to this he had a tendency to aestheticism which reveals itself in his physiological, geological, and geographical writings.

Humboldt's scientific reputation was chiefly due to his geographical work. He loved travel, and in pursuit of his scientific inquiries he visited Asia, Africa, and more particularly equatorial America. In his popular Ansichten der Natur he describes the impressions called forth by great mountains, by the river Amazon, by the primeval forest and the desert. The characteristics of each district are best described by the plants growing there; hence Humboldt gives in his book a plant physiognomy, dividing plants into nineteen groups, according to the impression they made. His groups are : The palms, which he regards as the tallest and most magnificent of plants ; the banana- like plants, with low-growing stems, rich in sap, which bear an apical crown of leaves ; the Malvaceae and Bombaceae plants with enormously thickened stems, the leaves heart-shaped or lobed, and covered with fine hairs; mimosas; moorland plants; cactus-like plants; orchids; casuarinas trees whose branches resemble, in their general habit of growth, the branches of an equisetum ; conifers ; forest trees with stems full of sap and with large leaves with prominent veins ; lianes ; aloes ; grass-like plants ; ferns ; liliaceous plants ; willow-like plants ; plants with the myrtle habit, with the laurel habit, &c.

Humboldt believed that the task of describing these forms was one for the landscape artist rather than for the scientist ; nevertheless the latter will find it an interesting exercise to examine this classification in greater detail, Humboldt's ideas are in fact still of value, although they receive little consideration. Another of Humboldt's attempts seems to have been completely forgotten, namely, his attempt to discover the causes of the present geographical distribution of plants and animals. Ten years before Darwin's work appeared Humboldt very strongly criticized

< those who like to dream of a gradual transformation of species, who look upon those cases where each island of a group has its own special parrot, as cases where there has been a modification of species ; . . . who attribute the wonderful uniformity of the above numbers (the numbers of species which inhabit the various geographical regions) to a migration of the same species into those regions ; this species having in the course of time been so changed by climatic conditions that in the end it has apparently replaced its pro- genitor.*

Why, then, have the common heather and the ordinary oak not spread eastwards across the Ural mountains ? Why are there no Rosaceae in the Southern Hemisphere, and why so few calceolarias in the Northern ? Humboldt gives an explanation of such facts of geographical distribu- tion. He believes that the number of species in each order is determined by a mathematical law, and that this law remains constant throughout any geological epoch. Once we have shown, as Humboldt showed, that in the Tem- perate zone -J of all phanerogams are Composites, ^ are grasses, ^ are Leguminoseae ; then, he asserted, it should be possible to take any one area viz. Germany to deter- mine the number of species belonging to any one of the above orders which occur in that area, and from this to make a rough estimate of the total number of flowering plants in the area, and of all the other groups of plants as well.

To take an example : There are many species of Gra- mineae, of Umbelliferae and Cruciferae, of Compositeae, Leguminoseae, and Labiateae, which are very common in Germany, but which do not occur in France ; and yet the relative numbers of the above-mentioned families are practically identical in the two countries. Hence the German species must be represented in France by other species of the same family. Agassiz devoted much time to investigating problems of geographical distribution, and arrived at a similar stand- point to Humboldt. He was convinced that there is some general law, or, as he would have expressed it, some Divine Idea, which determines the number of existing indi- viduals, species, and genera, as well as the limits of their distribution. He believed that there is a direct numerical relation between the number of carnivorous animals and the number of their prey. Some forms are very rare, others extremely common. This rarity or abundance is a charac-

teristic of the different forms, and is by no means due to mere chance. 2. Darwin's Historical and Experimental Conceptions applied to the Facts of Distribution. Darwin's ideas gave impetus to the study of biological aspects of geography. It was, indeed, a geographical fact which led him to consider the question of the origin of various species from a common ancestor, namely, the observation that the animal inhabitants of certain South American islands, though very similar, are yet not identi- cal. Darwin ascribed this similarity to the fact that the animals were descended from common ancestors, but had developed differently in different environments. His views were not related to those of Humboldt, however. He did not pay any attention to Humboldt's 'types', nor to his 'numerical law 5 , nor did he believe that there is any such simple relationship between the evolution of the organism and the area it usually inhabits. When he dis- cusses the geographical distribution of plants and animals he does not suggest any such direct interaction between them and their environment (that desert plants exhibit certain characteristic features, for example, while aquatic plants exhibit others). On the contrary, one of his leading themes is that the physical factors in the environment are not sufficient to explain the infinite variety of forms that inhabit these different areas. He suggests that some of the links which exist between the living organism and its surroundings can only be explained by an appeal to past history.

To give an illustration : The marsupials are only found in Australia and its adjacent islands and in South America. There are no causes, climatic or other, to which these facts of distribution can be satisfactorily attributed. But history gives the clue. Australia and South America were once connected by a land bridge which has long since disappeared. When this existed there was, however, no land connexion between North and South America. Hence similar animals are found even to-day in Australia and in South America animals which do not occur in North America.

Gingko is a peculiar tree which has often been cultivated of late years in Europe. It possesses only very distant relatives among living plants the modern conifers. The fact that this tree is found wild in China and Japan only can be best explained on historical grounds. This family of trees once contained many species, and was spread over the whole world. The family is dying out; Gingko is the last survivor, and is itself perhaps doomed to extinction.

Inspired by such ideas, Darwin asserted that the facts of geographical distribution can only be explained histori- cally, and that this is in itself a proof of the correctness of his theory. In actual practice he did not devote much attention to these historical explanations ; he merely cited them to prove that the present distribution of plants and animals over the surface of the globe is not due to some pre- determined plan. This argument was accepted, simply because his opponents were not inclined to attack it.

Darwin was always seeking for some secondary explana- tion of modern conditions. In pursuit of this end he focused his attention on the means by which an animal or plant is able to migrate from one district to another. He showed that some seeds retain their power of germina- tion, even after long immersion in water; that swamp birds carry eggs and seeds adhering to their feet from one swamp to another; that the wind, ocean currents, and even glaciers can carry organisms over long distances, and so make it possible for similar animals and plants to be found in very widely separated areas. He supposed that large stretches of ocean, climatic barriers or widespread deserts would hinder migration; hence these tend to become boundaries between animal and plant communities.

It was thus that Darwin analysed the facts, and he explained the present geographical distribution of animals just as we explain the present distribution of human races* Each species arose at some definite spot, as the offspring of one pair of parents. From that centre the descendants either spread out on all sides, so that the fatherland became the centre of the area inhabited by them, or they migrated into a new area. The prevailing distribution is the result of consolidation after long epochs, during which there was a continued and highly complicated succession of origins, migrations, and dispersals, all interwoven. Meanwhile sub-species were separated off from the parent species, and these became new and independent species in their turn. This is why the present distribution of animals and plants is so confused; this is why we meet nearly related forms in the same or in adjacent areas ; this is why any boundary between two areas is usually a boundary between living forms which have little in com- mon; for this reason high mountain chains, deep seas, long-established deserts are boundaries between quite distinct fauna and flora.

The fundamental assumption which underlies all Darwin's ideas about geographical distribution is that every species arose only once, and only in one place ; hence that it had only one ancestral home, from which it spread further afield. Darwin believed, further, that nearly related forms always arose near one another ; hence they inhabit neighbouring lands, unless they have wandered further afield in the course of migration. 3. Wallace's Zoological Regions and their Botanical Equivalents.

Wallace used the facts of animal distribution rather differently from Darwin, in deducing from them support for the theory of evolution. He did not pay much attention to the small and obvious agents by which existing organ- isms may possibly be transported from one part of the earth to another. His attention was focused upon the broad question of the modern boundaries between the various animal types, and he sought for the explanation of modern conditions in the geological records of the past. In 1857 Sclater had divided the world into regions accord- ing to the distribution of mammals. In close connexion with this work, Wallace divided the earth into six regions, each defined by the characteristic mammals inhabiting it :

The Palearctic realm, including the whole of Europe, the north of Asia as far as the central Himalayas, and Africa north of the Sahara. The Nearctic region, embracing all North America to the edge of the Mexican plateau. The Indian or Oriental region, consisting of Asia south of the Himalayas, and of the Sunda Islands (except Celebes and Lombok). The Ethiopian region, including Africa south of the Sahara, Southern Arabia, Madagascar, and the Mascarene Islands.

The Neotropical region, from the northern border of Mexico to Capetown. The Australian area, embracing Australia and the adjacent islands south-east from Celebes and Lombok (inclusive), together with New Zealand and Polynesia. Trouessart, the French geographer, added two more regions: the Arctic, including those northern areas with an average annual temperature which is below o C., and a similar Antarctic area. Wallace compared the forms inhabiting these different regions, to find which showed any anatomical relationship. From these facts, and from palaeontological evidence, he suggested avenues of migration along which the animal world had probably spread. In this work he assumed that each species had originated in one region only. (We have seen that Darwin made a similar assumption.) He showed that there is, for example, a marked analogy between the fauna of Europe, Asia, and North America. How was he to explain the fact that the same beaver is found in Europe and in America, that in both regions we find a similar bear or similar cattle (Bison europaeus and S. americanus), that very similar plants are also found in both ? Only by the assump- tion that formerly, in some not-far-distant geological epoch, the old world was united by a land bridge to the

new world, and that over this plants and animals had passed. For similar reasons Wallace assumed that at one time Africa was connected with Madagascar and Ceylon, on the one side, and with South America on the other. Wallace's method found many imitators, and, in par- ticular, it dominated zoological geography. It has extended, too, among botanists, and Drude's floristic areas became as important for botanists as were Wallace's regions for zoologists. Drude distinguished :

A. Oceanic Floras, which include those of the littoral, where marine algae and the so-called sea-grasses are found. 1. The Northern Temperate belt; Northern and Central Europe; Northern Asia; the most northerly regions of America. Here there occur woods, grasslands, tundras, and so on. 4. A Return to the Study of the Direct Dependence of the Flora and Fauna upon the Environment. In recent times the geographical methods of Darwin and Wallace have been gradually superseded. We have, however, retained their fundamental axiom, that some

knowledge of the facts of palaeontology is necessary before we can understand modern problems of distribution. But while general interest in such matters has waned, there has been a gradual revival of interest in the question of the direct relationship between the organism and its environment. This marks a return to the tendencies of pre- Darwinian times. Darwin's most characteristic assumption, that each species arose once and in one area only, is being challenged. Further, it is alleged that Darwin attached too much importance to the phenomena of migration. Migration did not consist so much in a definite leaving of the ancestral home as in a fluctuation here and there under the drive of climatic pressure ; the total effect would not lead to any great regional changes of fauna.