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1132 ZIRKEL, FERDINAND (1838–1912), German petrographer, was born at Bonn in 1838. He was educated at Bonn, where he trained as a mining engineer, but a journey he made to Iceland and the Faeroe Is., Scotland and England, and a meeting with Henry Clifton Sorby (see 25.431) attracted him to the study of microscopical petrography, then a comparatively new science. He became professor of mineralogy and geology successively at Lemberg (1863), Kiel (1868) and Leipzig (1870), retiring from the last-named post in 1909. He did much to develop the study of petrography, and his Lehrbuch der Petrographie (1866; 2nd ed. 1893–4) is a standard work on the subject. He was an hon. D.Sc. of Oxford, and also a foreign member of the Royal Society and an hon. member of the Mineralogical Society. He died at Bonn June 11 1912.

ZOBEIR RAHAMA (1830–1913), Egyptian pasha (see 28.992), died at Geili, near Khartum, Jan. 5 1913.

ZOÖLOGY (see 28.1022).—It is a sign of the vitality of a science such as zoölogy that its aspect should change from generation to generation. Paths of inquiry which are eagerly followed for one quarter of a century are sometimes almost forsaken during the next. There is change of emphasis and perspective. The reasons for this are mainly four, (1) A great discovery, such as Mendel made, opens up the possibility of conquering a territory hitherto unsubjugated, and this secures the enthusiasm of pioneers. (2) A new idea, such as that of discontinuous variations or mutations, alters the traditional outlook and is bound to affect zoological activity, e.g. taxonomy in the case mentioned; or a new scepticism, e.g. in regard to the transmission of exogenous somatic modifications ("acquired characters"), may prompt effort in reinterpreting facts. (3) A new contact, such as is suggested by the terms biochemistry and biophysics, biopsychology and psychobiology, always means a fresh series of formulations, making, in synthesis, for a more complete apprehension of that well-integrated unity—the Organism—which the various analytic methods seek to understand from different points of view. (4) A new method—such as section-cutting, differential staining, the use of the ultra-microscope, the utilization of statistics—may prove so rewarding that it attracts many recruits to its service, with the result that the trend of zoölogy is for a time markedly in that direction. There are other factors which change the aspect of zoölogy from generation to generation, such as the influence of great masters, like Huxley, Balfour, and Lankester; and the claims of the nation for scientific counsel, e.g. as regards agriculture, breeding, fisheries, and insect-pests. Moreover, the reflective intelligence of the scientific workers themselves, who have an ideal of the unity of their science, makes them tend, deliberately or subconsciously, to fill up gaps, as may be illustrated by the relatively recent development of the scientific study of animal behaviour.

For such reasons as have been indicated the aspect of zoölogyhas changed considerably since the opening of the 20th century,though it may be safely said that the things that remain much the same are of more importance than most of the novelties. Of some of the significant new departures, which led to notable results in the period 1910–21, account is taken in separate articles, e.g. under, , , , , , , ,, . What is attempted in the present article is a general survey, which may be conveniently arranged under the headings: Morphological, Physiological, Embryological and Aetiological (Evolutionary).

In its full scope morphology has to do with the static relations of organisms, with form and structure, as contrasted with physiology, which is concerned with the dynamical problems of vital activity. The two inquiries are obviously complementary and indispensable. It is a narrow view which affects to despise "purely morphological research"; as is obvious enough when any distinctively new type, like the sedentary ctenophore Tjalfiella, or the elusive Okapi, comes under consideration. Yet it must be admitted that zoölogical activity has in some measure turned aside from accumulating details of comparative anatomy at any and every level of analysis. As a discipline descriptive morphology is indispensable, but it is not to be persisted in too long, when there is so much else awaiting investigation. There is a widespread and not unreasonable opinion that morphological researches require some justification beyond their immediate result; they should contribute to our knowledge of affinities, of adaptations, of principles of architecture, and laws of change, as the best morphology has always done.

Causal Morphology.—The distinctive modern change has been the coming together of physiology and morphology, which developed for so long on paths that seldom even crossed. As E. S. Russell says, in his scholarly and critical Form and Function (1916), "Until well into the 'eighties animal morphology remained a purely descriptive science, content to state and summarize the relations between the coexistent and successive form-states of the same and of different animals. No serious attempt has been made to discover the causes which led to the production of form in the individual and in the race" (p. 314). The evolution theory offered only a formal solution, and for all practical purposes physiologists took the animal organization as given, not troubling about its mode of origin. Not a few saw the need of definitely tackling the physiology of development, notably Prof. W. His, in his well-known work, Unsere Korperform und das Problem ihrer Entstehung (1874), but the credit of founding a new sub-science of causal morphology (Entwicklungsmechanik) on an experimental basis certainly belongs to Prof. W. Roux, who has had many followers. The aim of Entwicklungsmechanik is defined by Roux to be " the reduction of developmental events to the fewest and simplest causal processes." Two classes of causal processes may be distinguished, as " complex compo- nents " and " simple components " of development. " The lat- ter are directly explicable by the laws of physics and chemistry; the former, while in essence physico-chemical, are yet so very complicated that they cannot at present be reduced to physico- chemical terms. . . . They represent biological generalizations, in their way of equal validity with the generalizations of physics and chemistry " (Russell, 1916, p. 319). They are, in fact, the general properties or functions of organized matter, such as assimilation and dissimilation, growth and reproduction, heredity and self-differentiation.

Biophysics. A special department of causal morphology may be usefully designated biophysics. As in regard to chemical processes, so in regard to physics, there is great promise in those investigations which carry up into the study of organisms the laws and lessons of the inorganic. An auspicious beginning of this difficult task has been made by Prof. D'Arcy W. Thompson, in his Growth and Form (1917), one of the foundations of biophysics. The aim of this inquiry is " to show that a certain mathematical aspect of morphology, to which as yet the morphologist gives little heed, is interwoven with his problems, complementary to his descriptive task, and helpful, nay, essential to his proper study and comprehension of form." Factors to be reckoned with are size, the principle of minimal areas, surface tension, equilibrium, the rate of growth in different directions, and the properties of colloids. The essay marks a big advance in the interpretation of form, whether in external structures like molluscan shells, the twisted horns of mammals and the shapes of eggs, or in internal architecture, like that of a bone, where statical and dynamical principles find fine illustration. Luminous also is Thompson's theory of transformations, in which it is shown how a single harmonious deformation may lead from one form of skull or leaf to that of a related type ; and how trammels or lines of constraint may determine the action of the expansive forces of growth, now in one direction and now in another. This is more than an approach to the principles of morphogenesis.

Let us state this step of progress concretely. Exact study has shown that the thigh-bone, for instance, is in its proportions and structure mechanically correct. It has great strength with a maximum economy of material, and the internal beams are in their thickness and closeness of spacing demonstrably adjusted