[439] _Studien z. Gastraea-Theorie_, p. 214, 1877. These forms were known even in 1870 (Carter, _Ann. Mag. Nat.
Hist._ (4), vi., pp. 346-7), to be Foraminifera. The figures of supposed collar-cells, etc., do credit to Haeckel"s imagination.
[440] _History of Creation_, Eng. Trans., ii., pp. 278 ff.
[441] _Systematische Phylogenie_, iii., p. 41, Berlin, 1895.
[442] "Notes on the Embryology and Cla.s.sification of the Animal Kingdom," _Q.J.M.S._ (n.s.), xvii., pp. 399-454, 1877.
[443] It was "part of the non-historic mechanism of growth" (_loc. cit._, p. 418).
[444] _Treatise on Comparative Embryology_, ii., chap.
xiii., 1881. For a modern discussion of this problem, see Hubrecht, _Q.J.M.S._, xlix., 1906.
[445] See Balfour, _loc. cit._, Chapter xiii.
[446] _A Treatise on Zoology_, Pt. ii., 1900. Introduction by Sir E. Ray Lankester.
[447] _Studien zur Blattertheorie_, Jena, 1879-80. "Die Coelomtheorie, Versuch einer Erklarung des mittleren Keimblattes," _Jenaische Zeitschrift_, xv., pp. 1-150, 1882.
[448] For an historical account of this work, see Lankester, _loc. cit._, pp. 21-37.
[449] _Proc. Roy. Soc._, 1883, and _Q.J.M.S._, xxiii., 1883.
[450] "Origin of Metameric Segmentation," _Q.J.M.S._, xxiv., pp. 43-82 1884.
[451] See further the same author"s article "Embryology"
in the _Ency. Brit._, vol. xi., 11th ed., Cambridge, 1910.
[452] _Arch. f. mikr. Anat._, xiii., pp. 181-204, 1877.
[453] "Der Bau von Gunda segmentata," _Mitth. Zool. Stat.
Neap._, iii., pp. 187-250, 1882.
[454] "Die Polycladen," _Fauna u. Flora des Golfes von Neapel_, Monog. v., Leipzig, 1884, and "Beitrage zu einer Trophocoeltheorie," _Jen. Zeits._, x.x.xviii., pp.
1-373, 1904 (which see for a modern account of theories of metamerism).
[455] "Die Abstammung der Anneliden u. Chordaten," _ Jen.
Zeits._, x.x.xix., pp. 151-76, 1905. "The Gastrulation of the Vertebrates," _Q.J.M.S._, xlix., pp. 403-19, 1906.
"Early Ontogenetic Phenomena in Mammals," _Q.J.M.S._, liii., pp. 1-181, 1909.
CHAPTER XVII
THE ORGANISM AS AN HISTORICAL BEING
"Of late the attempt to arrange genealogical trees involving hypothetical groups has come to be the subject of some ridicule, perhaps deserved. But since this is what modern morphological criticism in great measure aims at doing, it cannot be altogether profitless to follow this method to its logical conclusions. That the results of such criticism must be highly speculative, and often liable to grave error, is evident."
The quotation is from Bateson"s paper of 1886, and it is symptomatic of the change which was soon to come over morphological thought. New interests, new lines of work, began to usurp the place which pure morphology had held so long.
This is accordingly a convenient stage at which to take stock of what has gone before, to consider the relation of evolutionary morphology to the transcendental and the Cuvierian schools of thought which preceded it, and to make clear what new element evolution-theory added to morphology.
The close a.n.a.logy between evolutionary and transcendental morphology has already been remarked upon and ill.u.s.trated in the last three chapters.
We have seen that the coming of evolution made comparatively little difference to pure morphology, that no new criteria of h.o.m.ology were introduced, and that so far as pure morphology was concerned, evolution might still have been conceived as an ideal process precisely as it was by the transcendentalists. The principle of connections still remained the guiding thread of morphological work; the search for archetypes, whether anatomical or embryological, still continued in the same way as before, and it was a point of subordinate importance that, under the influence of the evolution-theory, these were considered to represent real ancestral forms rather than purely abstract figments of the intelligence. The law of Meckel-Serres was revived in an altered shape as the law of the recapitulation of phylogeny by ontogeny; the natural system of cla.s.sification was pa.s.sively inherited, and, by a _pet.i.tio principii_, taken to represent the true course of evolution. It is true that the attempt was made to subst.i.tute for the concept of h.o.m.ology the purely genetic concept of h.o.m.ogeny, but no inkling was given of any possible method of recognising h.o.m.ogeny other than the well-worn methods generally employed in the search after h.o.m.ologies.
There was a close spiritual affinity between the speculative evolutionists and the transcendentalists. Both showed the same subconscious craving for simplicist conceptions--the transcendentalists clung fast to the notion of the absolute unity of type, of the ideal existence of the "one animal," and the evolutionists did precisely the same thing when they blindly and instinctively accepted the doctrine of the monophyletic descent of all animals from one primeval form. Geoffroy persisted in regarding Arthropods as being built on the same plan as Vertebrates: Dohrn and Semper did nothing different when they derived both groups from an ancestor combining the main characters of both. The determination to link together all the main phyla of the animal kingdom and to force them all into a single mould was common to evolutionary and pre-evolutionary transcendentalists alike.
From the fact that all Metazoa develop from an ovum which is a simple cell, the evolutionists inferred that all must have arisen from one primordial cell. From the fact that the next step in development is the segmentation of the ovum, they argued that the ancestral Metazoa came into being through the division of the primal Protozoon with aggregation of the division-products. From the fact that a gastrula stage is very commonly formed when segmentation has been completed, they a.s.sumed that all germ-layered animals were descended from an ancestral Gastraea.
They quite ignored the possibility that a different explanation of the facts might be given; they seized upon the simplest and most obvious solution because it satisfied their overwhelming desire for simplification. But is the simplest explanation always the truest--especially when dealing with living things? One may be permitted to doubt it. It is easy to account for the structural resemblance of the members of a cla.s.sificatory group, by the a.s.sumption that they are all descended from a common ancestral form; it is easy to postulate any number of hypothetical generalised types; but in the absence of positive evidence, such simplicist explanations must always remain doubtful. The evolutionists, however, had no such scruples.
Phylogenetic method differed in no way from transcendental--except perhaps that it had learnt from von Baer and from Darwin to give more weight to embryology. The criticisms pa.s.sed by Cuvier and von Baer upon the transcendentalists and their recapitulation theory might with equal justice be applied to the phylogenetic speculations which were based on the biogenetic law. There was the same tendency to fix upon isolated points of resemblance and disregard the rest of the organisation. Thus, on the ground of a presumed a.n.a.logy of certain structures to the vertebrate notochord, several invertebrate groups, as the Enteropneusta, the Rhabdopleura, the Nemertea, were supposed to be, if not ancestral, at least offshoots from the direct line of vertebrate descent. And if other points of resemblance could in some of these cases be discovered, yet no successful attempt was made to show that the total organisation of any of these forms corresponded with that of the Vertebrate type.
With the possible exception of the Ascidian theory, all the numerous theories of vertebrate descent suffered from this irremediable defect, and none carried complete conviction.
In spite of the efforts of the evolutionists, as of those of the transcendentalists, the phyla or "types" remained distinct, or at best connected by the most general of bonds.
The close affinity of transcendentalists and evolutionists is shown very clearly in their common contrast in habits of thought with the Cuvierian school. It is the cardinal principle of pure morphology that function must be excluded from consideration. This is a necessary and unavoidable simplification which must be carried out if there is to be a science of pure form at all. But this limitation of outlook, if carried over from morphology to general biology becomes harmful, since it wilfully ignores one whole side of life--and that the most important. The functional point of view is clearly indispensable for any general understanding of living things, and this is where the Cuvierian school has the advantage over the transcendental--its principles are applicable to biology in general.
Geoffroy and Cuvier in pre-evolutionary times well typified the contrast between the formal and the functional standpoints. For Geoffroy form determined function, while for Cuvier function determined form. Geoffroy held that Nature formed nothing new, but adapted existing "materials of organisation" to meet new needs. Cuvier, on the other hand, was always ready to admit Nature"s power to form entirely new organs in response to new functional requirements.
The evolutionists followed Geoffroy rather than Cuvier. They laid great store by h.o.m.ological resemblances, and dismissed a.n.a.logies of structure as of little interest. They were singularly unwilling to admit the existence of convergence or of parallel evolution, and they held very firmly the distinctively Geoffroyan view that Nature is so limited by the unity of composition that she can and does form no new organs.
By no one has this underlying principle of evolutionary morphology been more explicitly recognised than by Hubrecht, who in his paper of 1887, after summarising the points of resemblance between Nemertines and Vertebrates which led him to a.s.sume a genetic connection between them, writes as follows:--"At the base of all the speculations contained in this chapter lies the conviction, so strongly insisted upon by Darwin, that new combinations or organs do not appear by the action of natural selection unless others have preceded, from which they are gradually derived by a slow change and differentiation.
"That a notochord should develop out of the archenteric wall because a supporting axis would be beneficial to the animal may be a teleological a.s.sumption, but it is at the same time an evolutional heresy. It would never be fruitful to try to connect the different variations offered, _e.g._, by the nervous system throughout the animal kingdom, if similar a.s.sumptions were admitted, for there would be then quite as much to say for a repeated and independent origin of central nervous systems out of indifferent epiblast just as required in each special case. These would be steps that might bring us back a good way towards the doctrine of independent creations. The remembrance of Darwin"s, Huxley"s, and Gegenbaur"s cla.s.sical foundations, and of Balfour"s and Weismann"s brilliant superstructures, ought to warn us away from these dangerous regions" (p. 644).
This same prejudice lies at the root of the idea of _Functionswechsel_, in spite of the general functional orientation of that idea.
Dohrn"s constant a.s.sumption is that Nature makes shift with old organs wherever possible, instead of forming new ones. He derives gill-slits from segmental organs, fins and limbs from gills, ribs from gill-arches, and so on, instead of admitting that these organs might quite as well have arisen independently. He objects on principle to the origin of organs _de novo_. Thus, reb.u.t.ting the suggestion that certain organs which are not found in the lower Vertebrates might have arisen as new formations, he writes:--"Against this supposition the whole weight of all those objections can be directed that are to be brought in general against the method of explanation which consists in appealing without imperative necessity to the _Deus ex machina_, "New formation," which is neither better nor worse than _Generatio equivoca_" (p. 21).
Of a similar nature was the objection to convergence.[456]
Why, we may ask, were morphologists so unwilling to admit the creative power of life? Dohrn, for instance, was fully aware of the great transforming influence exerted by function upon form--his theory of _Functionswechsel_ regards as the most powerful agent of change the activity of the animal, its effort to make the best use of its organs, to apply them at need in new ways to meet new demands. Why then did he not go a step further and admit that the animal could by its own subconscious efforts form entirely new organs? Why did most morphologists join with him in belittling the organism"s power of self-transformation?
The reasons seem to have been several. There is first the fundamental reason, that the idea of an active creative organism is repugnant to the intelligence, and that we try by all means in our power to subst.i.tute for this some other conception. In so doing we instinctively fasten upon the relatively less living side of organisms--their routine habits and reflexes, their routine structure--and ignore the essential activity which they manifest both in behaviour and in form-change.
We tend also to lay the causes of form-change, of evolution, as far as possible outside the living organism. With Darwin we seek the transforming factors in the environment rather than within the organism itself. We fight shy of the Lamarckian conception that the living thing obscurely works out its own salvation by blind and instinctive effort.
We like to think of organisms as machines, as pa.s.sive inventions[457]
gradually perfected from generation to generation by some external agency, by environment or by natural selection, or what you will. All this makes us chary of believing that Nature is prodigal of new organs.
Other causes of the unwillingness of morphologists to admit the new formation of organs are to be sought in the main principle of pure morphology itself, that the unity of plan imposes an iron limit upon adaptation, and in the powerful influence exercised at the time by materialistic habits of thought. Teleology had become a bugbear to the vast majority of biologists, and all real understanding of the Cuvierian att.i.tude seems, in most cases, to have been lost, although, curiously enough, teleological conceptions were often unconsciously introduced in the course of discussions on the "utility" of organs in the struggle for existence.
Evolutionary morphology, being for the most part a form of pure or non-functional morphology, agreed then in all essential respects with pre-evolutionary or transcendental morphology.
But it contained the germ of a new conception which threw a new light upon the whole science of morphology. This was the conception of the organism as an historical being.