Form and Function Part 26

CHAPTER XIV

ERNST HAECKEL AND CARL GEGENBAUR

At the time when Darwin's work appeared there already existed, as we have seen, a fully formed morphology with set and definite principles.

The aim of this pre-evolutionary morphology had been to discover and work out in detail the unity of plan underlying the diversity of forms, to disentangle the constant in animal form and distinguish from it the accessory and adaptive. The main principle upon which this work was based was the principle of connections, so clearly stated by Geoffroy.

The principle of connections served as a guide in the search for the archetype, and this search was prosecuted in two directions--first, by the comparison of adult structure; and second, by the comparative study of developing embryos. It was found that the archetype was shown most clearly by the early embryo, and this embryological archetype came to be preferred before the archetype of comparative anatomy. It became apparent also that the parts first formed (germ-layers) were of primary importance for the establishing of h.o.m.ologies.

While practically all morphologists were agreed as to the main principles of their science, they yet showed, as regards their general att.i.tude to the problems of form, a fairly definite division into two groups, of which one laid stress upon the intimate relation existing between form and function, while the other disregarded function completely, and sought to build up a "pure" or abstract morphology. In opposition to both groups, in opposition really to morphology altogether, a movement had gained strength which tended towards the a.n.a.lysis and disintegration of the organism. This movement took its origin in the current materialism of the day, and found expression particularly in the cell-theory and in materialistic physiology.

The separation between morphology as the science of form and physiology as the science of the physics and chemistry of the living body had by Darwin's day become well-nigh absolute.

The morphology of the 'fifties lent itself readily to evolutionary interpretation. Darwin found it easy to give a formal solution of all the main problems which pre-evolutionary morphology had set--he was able to interpret the natural system of cla.s.sification as being in reality genealogical, systematic relationship as being really blood-relationship; he was able to interpret h.o.m.ology and a.n.a.logy in terms of heredity and adaptation; he was able to explain the unity of plan by descent from a common ancestor, and for the concept of "archetype" to subst.i.tute that of "ancestral form."

The current morphology, Darwin found, could be taken over, lock, stock and barrel, to the evolutionary camp.

In what follows we shall see that the coming of evolution made surprisingly little difference to morphology, that the same methods were consciously or unconsciously followed, the same mental att.i.tudes taken up, after as before the publication of the _Origin of Species_.

Darwin himself was not a professional morphologist; the conversion of morphology to evolutionary ideas was carried out princ.i.p.ally by his followers, Ernst Haeckel and Carl Gegenbaur in Germany, Huxley, Lankester, and F. M. Balfour in England.

It was in 1866 that Haeckel's chief work appeared, a _General Morphology of Organisms_,[366] which was intended by its author to bring all morphology under the sway and domination of evolution.

It was a curious production, this first book of Haeckel's, and representative not so much of Darwinian as of pre-Darwinian thought. It was a medley of dogmatic materialism, idealistic morphology, and evolution theory; its sources were, approximately, Buchner, Theodor Schwann, Virchow, H. G. Bronn, and, of course, Charles Darwin.

It was scarcely modern even on its first appearance, and many regarded it, not without reason, as a belated offshoot of _Naturphilosophie_.

Its materialism is of the most intransigent character. The form and activities of living things are held to be merely the mechanical result of the physical and chemical composition of their bodies. The simplest living things, the Monera, are nothing more than h.o.m.ogeneous ma.s.ses of protein substance. "They live, but without organs of life; all the phenomena of their life, nutrition and reproduction, movement and irritability, appear here as merely the immediate outcome of formless organic matter, itself an alb.u.men compound" (p. 63, 1906).

Teleology, the Achilles' heel of Kant's (otherwise sound!) philosophy, is to be regarded as a totally refuted and antiquated doctrine, definitely put out of court by Darwinism.

Haeckel works out his materialistic philosophy of living things very much after the fashion of Schwann. There is the same talk of cells as organic crystals, of crystal trees, of the a.n.a.logy between a.s.similation by the cell and the growth of crystals in a mother liquid. Heredity and adaptation are shown equally as well by crystals as by organisms; for heredity, or the internal _Bildungstrieb_ (!), is the mechanical effect of the material structure of the crystal or the germ, and adaptation, or the external _Bildungstrieb_, is a name for the modifications induced by the environment. Adaptation so defined comes to be synonymous with the fortuitous variation which plays so great a part in Darwin's theory of natural selection.

It goes without saying that Haeckel allowed to the organism no other nor higher individuality than belongs to the crystal, and took no account at all of that harmonious interaction of the organs which Cuvier called the principle of the "conditions of existence." The concept of correlation had simply no meaning for Haeckel. The a.n.a.lysis and disintegration of the organism was pushed by him to its logical extreme, and in this also he was a child of his time.

A no less important influence clearly visible in the _General Morphology_ is the idealistic morphology of men like K. G. Carus and H. G.

Bronn. In previous chapters we have seen how K. G. Carus attempted to work out a geometry of the organism, and how Bronn tried in a modest way to found a stereometrical morphology, but had the grace not to push his stereometry _a l'outrance_, recognising very wisely that the greater part of organic form is functionally determined. Haeckel took over this idea[367] and pushed it to wild extremes, founding a new science of "Promorphology" of which he was the greatest--and only--exponent.[368]

This "science" dealt with axes and planes, poles and angles, in a veritable orgy of barbarous technical terms. It was intended to be a "crystallography of the organic," and to lay the foundations of a mechanistic morphology, or morphography at least.

How it was to be linked up with the physics and chemistry of living matter on the one hand and with the ordinary morphology of real animals on the other, was never made quite clear.

The science of Promorphology has no historical significance; it is interesting only because it ill.u.s.trates Haeckel's close affinity with the idealistic morphologists.

Another abortive science of Haeckel's, the science of Tectology, was equally a heritage from idealistic morphology. Tectology is the science of the composition of organisms from individuals of different orders.

There were six orders of individuals:--(1) Plastids (Cytodes and cells); (2) Organs (including cell-fusions, tissues, organs, organ-systems); (3) Antimeres (h.o.m.otypic parts, _i.e._, halves or rays); (4) Metameres (h.o.m.odynamic parts, _i.e._, segments); (5) Persons (individuals in the ordinary sense); (6) Corms (colonial animals).

The thought is essentially transcendental, and recalls the "theory of the repet.i.tion of parts," of which so much use was made by the German transcendentalists, such as Goethe,[369] Oken, Meckel and K. G. Carus, as well as by Duges.

The third, and naturally the most important, ingredient in the _General Morphology_ was the doctrine of evolution, in the form given to it by Darwin. We have here no concern with Haeckel's evolutionary philosophy, with the way in which he combined his evolutionism and his materialism to form a queer Monism of his own. We are interested only in the way he applied evolution to morphology, what modifications he introduced into the principles of the science, and in general in what way he interpreted the facts and theories of morphology in the light of the new knowledge.

We find that he repeats very much what Darwin said, giving, of course, more detail to the exposition, and elaborating, particularly in his recapitulation theory or "biogenetic law," certain doctrines not explicitly stated by Darwin.

Like Darwin he held that the natural system is in reality genealogical.

"There exists," he writes, "one single connected natural system of organisms, and this single natural system is the expression of real relations which actually exist between all organisms, alike those now in being on the earth and those that have existed there in some past time.

The real relations which unite all living and extinct organisms in one or other of the princ.i.p.al groups of the natural system, are genealogical: their relationship in form is blood-relationship; the natural system is accordingly the genealogical tree of organisms, or their genealogema.... All organisms are in the last resort descendants of autogenous Monera, evolved as a consequence of the divergence of characters through natural selection. The different subordinate groups of the natural system, the categories of the cla.s.s, order, family, genus, etc., are larger or smaller branches of the genealogical tree, and the degree of their divergence indicates the degree of genealogical affinity of the related organisms with one another and with the common ancestral form" (ii., p. 420).

The degree of systematic relationship is thus the degree of genealogical affinity. It follows that the natural system of cla.s.sification may be converted straightway into a genealogical tree, and this is actually what Haeckel does in the _General Morphology_. The genealogical trees depicted in the second volume (plates i.-viii.) are nothing more than graphic representations of the ordinary systematic relationships of organisms, with a few hypothetical ancestral groups or forms thrown in to give the whole a genealogical turn.

If the genealogical tree is truly represented by the natural system, it would seem that for each genus a single ancestral form must be postulated, for each group of genera a single more primitive form, and so in general for each of the higher cla.s.sificatory categories, right up to the phylum. Species of one genus must be descended from a generic ancestral form, genera of one family from a single family _Urform_, and so on for the higher categories.

This consequence was explicitly recognised by Haeckel. "Genera and families," he writes, "as the next highest systematic grades, are extinct species which have resolved themselves into a divergent bunch of forms (_Formenbuschel_)" (ii., p. 420).

The archetype of the genus, family, order, cla.s.s and phylum was thus conceived to have had at some past time a real existence.

The natural system of cla.s.sification is based upon a proper appreciation of the distinction between h.o.m.ological and a.n.a.logical characters.

Haeckel, following Darwin, naturally interprets the former as due to inheritance, the latter as due to adaptation, using these words, we may note, in their accepted meaning and not in the abstract empty sense he had previously attributed to them.[370] Similarly the "type of organisation," in von Baer's sense, was due to heredity, the "grade of differentiation" to adaptation.

So far Haeckel merely emphasised what Darwin had already said in the _Origin of Species_. But by his statement of the "biogenetic law," and particularly by the clever use he made of it, Haeckel went a step beyond Darwin, and exercised perhaps a more direct influence upon evolutionary morphology than Darwin himself.

Haeckel was not the original discoverer of the law of recapitulation. It happened that a few years before the publication of Haeckel's _General Morphology_, a German doctor, Fritz Muller by name, stationed in Brazil, had been working on the development of Crustacea under the direct inspiration of Darwin's theory, and had published in 1864 a book[371] in which he showed that individual development gave a clue to ancestral history.

He conceived that progressive evolution might take place in two different ways. "Descendants ... reach a new goal, either by deviating sooner or later whilst still on the way towards the form of their parents, or by pa.s.sing along this course without deviation, but then instead of standing still advancing still farther" (Eng. trans., p.

111). In the former case the developmental history of descendants agrees with that of the ancestors only up to a certain point and then diverges.

"In the second case the entire development of the progenitors is also pa.s.sed through by the descendants, and, therefore, so far as the production of a species depends upon this second mode of progress, the historical development of the species will be mirrored in its developmental history" (p. 112).

Of course the recapitulation of ancestral history will be neither literal nor extended. "The historical record preserved in developmental history is gradually _effaced_ as the development strikes into a constantly straighter course from the egg to the perfect animal, and it is frequently _sophisticated_ by the struggle for existence which the free-living larvae have to undergo" (p. 114).

It follows that "the primitive history of a species will be preserved in its developmental history the more perfectly the longer the series of young stages through which it pa.s.ses by uniform steps; and the more truly, the less the mode of life of the young departs from that of the adults, and the less the peculiarities of the individual young states can be conceived as transferred back from later ones in previous periods of life, or as independently acquired" (p. 121).

Applying these principles to Crustacea, he concluded that the shrimp _Peneus_ with its long direct development gave the best and truest picture of the ancestral history of the Malacostraca, and that accordingly the nauplius and the zoaea larvae represented important ancestral stages. He conceived it possible so to link up the various larval forms of Crustacea as to weave a picture of the primeval history of the cla.s.s, and he made a plucky attempt to work out the phylogeny of the various groups.

The thought that development repeats evolution was already implicit in the first edition of the _Origin_, but the credit for the first clear and detailed exposition of it belongs to F. Muller.

In much the same form as it was propounded by Muller it was adopted by Haeckel, and made the corner-stone of his evolutionary embryology.

Haeckel gave it more precise and more technical formulation, but added nothing essentially new to the idea.

It is convenient to use his term for it--the biogenetic law (_Biogenetische Grundgesetz_)--to distinguish it from the laws of Meckel-Serres and von Baer, with which it is so often confused.

Haeckel's statement of it may best be summarised in his own words, "Ontogeny, or the development of the organic individual, being the series of form-changes which each individual organism traverses during the whole time of its individual existence, is immediately conditioned by phylogeny, or the development of the organic stock (phylon) to which it belongs.

"Ontogeny is the short and rapid recapitulation of phylogeny, conditioned by the physiological functions of heredity (reproduction) and adaptation (nutrition). The organic individual (as a morphological individual of the first to the sixth order) repeats during the rapid and short course of its individual development the most important of the form-changes which its ancestors traversed during the long and slow course of their palaeontological evolution according to the laws of heredity and adaptation.

"The complete and accurate repet.i.tion of phyletic by biontic development is obliterated and abbreviated by secondary contraction, as ontogeny strikes out for itself an ever straighter course; accordingly, the repet.i.tion is the more complete the longer the series of young stages successively pa.s.sed through.

"The complete and, accurate repet.i.tion of phyletic by biontic development is falsified and altered by secondary adaptation, in that the bion[372] during its individual development adapts itself to new conditions: accordingly the repet.i.tion is the more accurate the greater the resemblance between the conditions of existence under which respectively the bion and its ancestors developed" (ii., p. 300).

The last two propositions, it will be observed, are taken over almost verbally from F. Muller.

Now we have seen that the natural system of cla.s.sification gives a true picture of the genealogical relationships of organisms, that the smaller and larger cla.s.sificatory groups correspond to greater or lesser branches of the genealogical tree. If ontogeny is a recapitulation of phylogeny, we must expect to find the embryo repeating the organisation first of the ancestor of the phylum, then of the ancestor of the cla.s.s, the order, the family and the genus to which it belongs. There must be a threefold parallelism between the natural system, ontogeny and phylogeny (ii., pp. 421-2).