The metamorphoses of animals, especially those of insects, were in the beginning described by Lyonnet„ Rocsel, De Geer. Retaumur, and others, and in recent times by Fritz Mfiller, who pointed out the fact that the caterpillar and maggot form of larva are secondary, the most gen eralized and primitive insects not undergoing a metamorphosis. Miller also worked out the metamorphosis of certain Crust:teen, and our knowledge of the larval forms of these animals has been greatly extended by Dohrn, Brooks, Smith, and others, with the result that the nau plius stage is now regarded as also an acquired one. and not a primitive feature. The most notable advance in our knowledge of the exact mode of metamorphosis in the most highly spe cialized insects, especially the flies, was made by Weismann in 18(14. His demonstration of the making over of the body by the histolysis of the larval tissues, and the formation of the tissues of the adult from imaginal buds. brought about the final overthrow of the preformation notion. Through the labors of Barrande, Beecher. and others, the larval stage of trilobites and of j..imu lus is recognized to be a third arthropod larval type, the protaspis, perhaps the most primitive and trochosphere-like of all.
The problem of the ancestry of insects, the so lution of which is based on our knowledge of larval forms and of the anatomy of primitive types. has been attacked by Brauer. Packard, Lubbock, Sylvestri. and others. Indeed, the latest results of work on the arthropods tends to show that this vast assemblage of forms is polyphyletic, and recently it has been subdivided into several phyla, each having a separate origin from some unknown worm allied to existing an nelids. and each with a different type of struc ture. The heterogeneous group of 'ernes 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 knowledge of the efflo mie o• '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 zonlogy and medical skill. Here likewise the dis covery of phagocytes by Aletschnikoff, also by Rowalevsky, and of their action in destroying dis ease-germs or bacteria, and Aletschnikoff's theory of inflammation, 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, Gegenbanr, 0. Hertwig, and others. While Huxley demolished the old view that the verte brate skull consists of modified vertebra?, the shill 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 (Ivo 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 Schrader. 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 limbs. Gegenbaur derived them from the archi pterygion or primitive portions of the shark's fins; while Thatcher. Mivart. Balfour. Holm]. and Wiedersheim have traced them back to the two primitive fin-folds of the lowest vertebrates. Here also belongs Dolirn's principle of eliange of function. one lying at the basis of the mechanical evolution of limbs and other organs. Another re sea reh started by Baldwin Spencer is the origin of the pineal gland and its nature as a third or median eye, a feature with little dm id inherited from amphioxus by the ganoid fishes, amphibia, reptiles, mammals, and even man, whose so-called pineal holly or gland, was by the ancient anat omists supposed to be the seat of the soul. Its complex homologies were indicated by Gaup in 1901. And here might be mentioned the great ad vances in the auatomy of the brain both of arthro pods and of vertebrates, particularly the recent thorough-going researches of Golgi on the rods of Corti, and of Ramon-y-01in] on the minute anatomy of the cerebellum; all such very special work laying the foundations for modern physiol ogy and psychology. and resulting in the neurone theory established between 1890 and 1900.
On the whole the most remarkable single dis covery in zoi)log,y, and one of great moment to anthropology. is the detection of the remains of the Pitheeanthropus by Dubois (in 1891-92) the late Tertiary strata of Java.
The aim and methods of modern 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, hut also the gradual, and at times sudden, modifications and evolution of series of types 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 .harsh in America. Other groups, as the Camelidie, etc., have had their ancestry elneidated by Cope, Osborn, Scott, Wortman, and others. The phylogeny of the ammonites and nautiloids has been elaborated by Hyatt, that of Brachiopoda by Beecher and others.