PALEONTOLOGY
413
PALEONTOLOGY
general until the era of the worms. One family of
worms already had gills, yet it was only upon the
appearance of the tnoUuscoidea that such organs for
breathing were always present. In the same manner
the crocodiles, alone of the reptilia, have a heart di-
vided into two ante-chambers and two main cham-
bers, a form of heart which is found, once more, with-
out exception among birds and mammals. This agree-
ment among various groups, however, cannot be based
upon a close relationship, but, strictly speaking, comes
also under the conception of convergence.
If we survey extinct organisms, there are without doubt many important considerations which tell for the theory of development. However, the theory of development in its extreme, monistic sense, signifies that all life, both animal and plant, springs from a sin- gle root. For this many proofs are still lacking, even if we set aside the fact that the oldest organisms of every family (except the vertebrates and plants) are highly organized, inasmuch as their oldest progenitors may have been made unrecognizable by the metamor- phosis of the earliest rocks and thus withdrawn from our observation; and even if the enormous length of time required for the development of forms so highly speciahzed as the trilobite, does not seem to be suffi- ciently represented in the eozoic sediments. But in the later formations also the entire family of verte- brates appear without any preparation; among the plants to name only a few, the flowering cretaceous angiosperms appear without any precursors, and the Older Tertiarj' brings without warning us, all ten or- ders of the mammalia; even among these ten orders a closer relationship can be conjectured in only a few cases. In the pedigree of organic beings, therefore, we meet with chasms which cannot be bridged over even with the help of Hackel's metakinesis. In view of this fact it is hardly possible any longer to maintain the opinion that all life has sprung from a single root (monophyletic). It appears much more probable that the different genera of animals and plants originate in various roots (polyphyletic). The advocates of the monophyletic theory, it is true, declare that the experi- ence of animal breeders and florists shows that new variations appear for the first time in few examples only, and that in view of the fragmentary character of palaeontological records these first examples may have perished. If we were to accept this explanation we should deceive ourselves as to the difficulties of the problem of development. For in every case a whole series of intermediate links is missing, and it would, therefore, be strange that none of these should have been transmitted to us. It would be still more start- ling if the transition-links had regularly perished in all the larger units of classification.
We infer therefore that the facts presented to us by the known fossils compel us to accept a polyphyletic descent. It is, therefore, interesting that zoologists like E. von Beer, I'Icisi'limann, and Th. Boveri, and a num- ber of bolaiiists Hkc A. von Kerner, who work in a different held, have also gradually adopted a polj'phy- letic line of descent.
Finally, if we examine more closely the individual groups of forms, we see their mutual relations in a new and peculiar light. For the studies in question show tliat tlic extinct animals and plants, while differing more or less in structure from those now living, did not fall below them in the perfection of their organization, that, on the contrary, in many cases indeed, a decline is manifested. All the great orders begin at once with Iiii:lily liilTi ii-ntiated forms, so that, with Koken, we can 'iiilv -peak of a "modification of limited system- atic Jivi.--ions".
Development may, therefore, take place \\ithout progress in organizations, for all forms which have been classified as belonging to the same genus or the same family stand upon the same level of organization. The difference consists essentially in a strong differ-
entiation and speciahzation of peculiarities, which are
subject now to an increase and again to a decrease.
Bj' means of this metamorphosis new species, new
genera, and even new famihes may easily arise. This
may exemplify for us progressive development, which,
however, should be strictly distinguished from ascend-
ing development. The new forms produced to-day in
the breeding of animals or in floriculture, belong en-
tirely to the domain of progressive evolution. Hith-
erto unquestioned proofs of ascending development
have been lacking in pala?ontology, nor does experi-
ment supply the deficiency. We may therefore say
that the organisms of the geological ages are connected
by descent, and that there is good reason for accepting
progressive development in the several lines of de-
scent down to the present time. But if we go beyond
this and set up a divergent line of descent for the
whole world of organisms, or seek to trace all organ-
isms back to a single cell, we abandon the foundation
of fact. If, therefore, we infer that a general develop-
ment cannot be established by the facts, we are still
within the lines of the theory of descent, for the essen-
tial conception of this theory is that the systematic
species of zoology and botany are not rigid and un-
changeable, but have developed from ancestors unlike
themselves, and may likewise develop into flifferently
formed descendants. It is the business of the theory
of deve'opment to investigate the facts and causes
which underlie the series of organic forms, at the head
of which stand existing species. Consequently, it is no
e.ssential part of its aim to prove that development is
ascending or that it supposes a single original progeni-
tor.
One of the questions involved in this problem is that of the descent of man, which will be touched on here because it has aroused the greatest interest. We may begin by stating that pala;ontology has, indeed, made known to us an older race of men with very beetling brows and an almost total absence of chin, but that up to now no ape-like progenitors of men have been dis- covered. Wherever fossil remains of man have been found — and hitherto they have been found only in the Quaternary period, for all reports of Tcrtiarj- man have so far been proved unreliable — man always ap- pears as a true man. So far only a relatively small number of remains of Quaternary man are known (e. g. the skulls of Spy, Neandertal, and Krapina, and the lower jaws of Schipka, La Naulctte, and Ochos). There is, moreover, the Pilhecanlhropus eredus, parts of the skeleton of which were found by the Dutch military surgeon Eugen Dubois in 1891 on the island of Java. Since its discovery it has been industriously brought forward by certain supporters of the theory of development as the long-sought missing link between ape and man. At present, however, it is agreed that this Pilhecanlhropus is only a large gibbon, an ape, al- though there is no doubt that, as regards the size of brain, he should be placed between the largest man- ape now known and man. One more fact must be emphasized. Volz and Elbert have Litely investigated the locality in Java where the I'llln ninihinjins w.as found, and they have proved inconte.-lalilv that the strata in which these remains were discovered belong to the (Quaternary period, that therefore the Pilhrcini- thropus crcctus was a contemporary of man and could not be his ancestor.
When we look at Hackel's "Stammbaum der Primaten" (Descent of the Primates), the pedigree seems somewhat fuller. In this work the ancestors of man are arranged in the following order: Archi- primas, from which are descended the Pacliytemures, including the Lemuraviilir, from which in turn the necrolemures are descended; and these are the direct ancestors of the apBs. Starting with the ape the descent is continued as follows: Archipilhecus, the primeval ape: Prothylnhates, the primeval gibbon; Pithecanthropus alalus, the speechless man-ape; Homo
