Page:The New International Encyclopædia 1st ed. v. 20.djvu/932

* ZOOLOGY. "94 lus is recognized to be a tliird arthropod larval tvpc, the protaspis, jierliaps the roost primitive and troehosphere-like of all. The problem of the ancestry of insects, the so- lution of which is based on our Icnowledge of larval forms and of the anatonij' of primitive types, has been attacked by Erauer, Packard, Lubbock, Sylvestri, and others. Indeed, the latest results of work on the arthropods tends to show that this vast assemblage of forms is polyphyletie, and recently it has l)een subdivided into several phyla, each having a separate origin from some unknown worm allied to existing an- nelids, and each with a difl'erent type of struc- ture. Tile lieterogeneous group of Vermes has also been disintegrated into several phyla. These changes have in great part been necessitated by the advance made by the two Hertwigs, Lan- kester, and others in our Icnowledge of the copIo- mie or bod.y cavity of different zoological groups. Fresh attention is now being paid to a study of the life-history of the Sporozoa (q.v. ). The story of the manner in which the germs are conveyed from one host to another, told by Grassi. Ross, and others, is one of the triumphs of modern zoiilogy and medical skill. Here likewise the dis- covery of " phagoc.rtes by Metschnikotf. also by Kowalevsky, and of their action in destroying dis- ease-germs or bacteria, and Metschnikoffs theory of intiammation, are among the most notable ad- vances made in the nineteenth century. As to vertebrate morphology the most impor- tant advances made during the last half of the nineteenth century are those contributed by Hux- ley, Gegenbaur, 6. Hertwig, and others. While Huxley demolished the old view that the verte- brate skull consists of modified vertebrse, the skull elements being in reality dermal bones de- rived from scales, the German anatomists named, as well as Dohrn. Loey, and others, have shown that the head of vertebrates is made up of seg- ments, as proved by the actual segments revealed in the embryo of the lower vertebrates, by the segmental arrangement of the muscles (myo- tomes), of the arteries, and of the cranial nerves. Apropos of the segmental arrangement of the organs of the vertebrate head, Loeb, as well as Sehradcr, from the point of view of experimental physiology, claim that no localization of func- tions exists, either in the brain or in the spinal cord of the cold-blooded vertebrates, and that the reactions observed are "only segmental reflexes, just as in the annelids and arthropods." Another problem attacked by morphologists in late years is that of the origin of vertebrate limlis. Gegenbaur derived them from the archi- pterygi(m or primitive portions of the shark's fins; while Thatcher. IMivart. Balfour. Dohrn. and Wiedersheim have traced them back to the two primitive fin-folds of the lowest vertebrates. Here also belongs Dohrn's prinei])le of change of function, one lying at the basis of the mechanical evolution of limbs and other organs,. othor re- search started by Baldwin S])encer is the origin of the ])ineal gland and its nature as a third or median eye. a feature with li'itle doubt inherited from amphio.xus by the ganoid fishes, amphibia, reptiles, mammals, and cAcn man, whose so-called pineal body or gland, was by the ancient anat- omists supposed to be the seat of the soul. Its complex homologies were indicated by Gaup in ZOOLOGY. 1901. And here might be mentioned the great ad- vances in the anatomy of the brain both of arthro- pods and of vertebrates, particularly the recent tliorough-going researches of Golgi on the rods of Corti, and of Ramon-y-C'ajal on (he minute anatomy of the cerebellum; all such very special work laying the foundations for modern physiol- og}' and psychologv'. and resulting in the neurone theory established" between 1 890 and 1900. On the whole the most remarkable single dis- covery in zoology, and one of great moment to anthropology, is the detection of the remains of the Pithecanthropus by Dubois (in 1S91-92) ia the late Tertiary strata of Java. The aim and methods of modem paleontology, which is in reality the study of the ancestors of the forms now living, has come to be one of the main supports of the theory of descent. And this not only for the reason that it has revealed to us the remains of numerous extinct groups, families, and orders, connecting those previously supposed to be wide apart, but because our museums now contain rich series of forms illustrating not only the geological succession, but also the gradual, and at times sudden, modifications and evolution of series of t,y])es from the generalized to the specialized. The chief factors also of this process of divergent evolution are quite clearly seen to have been geological and climatic changes, and the use and disuse of organs. Examples are the genealogy of the horse family, partly worked out by Huxley, Gaudry, Kowalevsky in Europe, and more perfectly, owing to the greater completeness of the series, by ilarsh in America. Other groups, as the Camelida?, etc., have had their ancestry elucidated by Cope. Osbom, Scott, Wortman, and others. The phylogeny of the ammonites and nautiloids has been elaborated by Hyatt, that of Brachiopoda by Beecher and others. Among the Crustacea, two orders, the Phyllo- carida and the Svncariila. have been recognized by Packard, and the latter group also recognized by Ortman. as being generalized fonns which have given rise to the higher more s])ecialized groups represented by the modern shrimjis, lobster, and crabs. Perhaps the most important paleontolo^ical discovery in the last half century, as regards arUiropod animals, is that of the appendages of trilobites. which we owe to Waleott, Matthew, and especially to Beecher. Finally, the view has of late years been rendered clear that nearly every class of invertebrate animals originated in Prccambrian times: the vertebrates with limbs and lungs. Amphibians and rejitiles, appearing near the close of the Paleozoic Age. while birds and mammals diverge from some reptilian stock in the early Mesozoie. ( 4 ) The fourth period is that of evolution and bionomics. The history of evolution (q.v.) is now a twice-told tale. How much we are indebted to Darwin (q.v.), Wallace (q.v.), Herbert Spencer (q.v.), and others has been re- counted elsewhere. H remains only to point out some salient features in the histiuT of the theory of descent and its modiliciitions during the nineteenth century. A noticeable circum- stance in the history of the rise and spread of the doelrine is the way in which the once almost forgotten or misun<lerstood views of Lamarck (q.v.) have been either unearthed or hit upon