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1134 a new type with real taxonomic importance comes every now and then as the reward of drudgery. Nor can it be doubted that an evolutionary study of species, especially when it can be linked to genetic studies of the living material, may lead us nearer the main object in view the understanding of life. Educationally, it may be doubted if the Linnaean discipline of species-making can be dispensed with without serious loss of efficiency.

Palaeonlological Study. The differentia of palaeontology is that it studies organisms in their "time relations, in their genetic sequence from age to age. A taxonomy based only on a study of forms extant " a horizontal section across the tree of life " may be shrewd, but it is incomplete without the work of the palaeontologist who exposes the relations of the buried branches. It is certain that palaeontological research has done well by zoology in this respect, as may be well illustrated by the later work of Prof. D. M. S. Watson on the taxonomy of amphibians, reptiles and mammals, or by the magnificent history of the extinct Equidae which has crowned the labours of Prof. H. F. Osborn.

Another achievement of the palaeontologists has been their increasingly satisfactory vindication of evolution as a historic process. Thus, to quote Dr. F. A. Bather (1920, p. 67): " if we take a chronological series of apparently related species or mutations, a 1, a 2 , a 3 , a 4 , and if in a 4 we find that the growth- stage immediately preceding the adult resembles the adult a 3 , and that the next preceding stage resembles a 2 , and so on; if this applies, mutatis mutandis, to other species of the series; and if, further, the old age of each species foreshadows the adult character of its successor ; then we are entitled to infer that the rela- tion between the species is one of descent." This method of prov- ing filiation is clearly illustrated by work on Ammonites. " Thus, large ammonites of the Xipheroceras planicosta group, beginning smooth, pass through a ribbed stage, which may be omitted, through unituberculate and bituberculate stages, back to ribbed and smooth again " (Bather, 1920, p. 68).

It is clear that taxonomic categories based on the study of existing forms must be supplemented by new categories dis- closed by the palaeontologists' study of change in time. " Thus many crinoids with pinnulate arms arose from others in which the arms were non-pinnulate. We cannot place them in an order by themselves, because the ancestors belonged to two or three orders. We must keep them in the same orders as their repre- sentative ancestors, but distinguish a Grade Pinnata from a Grade Impinnata " (Bather, 1920, p. 63).

To the palaeontologists are due, we think, convincing proofs that evolutionary change may be sometimes continuous, by gradual transition rather than by saltation. In their contri- butions to such problems as orthogenesis, the rise and decline of species, the tempo of evolution, and the correlation of organic with cosmic changes, the palaeontologists have certainly kept pace with the " neontologists."

Animal Histology. Without unduly trespassing on the arti- cle CYTOLOGY, we may indicate some general features of recent progress in animal histology, (a) The concept of the cell has become more fluid; for we recognize cells without very definite limits living in syncytia, cells with protoplasmic bridges binding them to their neighbours, cells with nuclear dust instead of a nucleus, and so on. Many zoologists agree with Prof. Clifford Dobell that to speak of a Ciliated Infusorian (let us say) as a single cell is more misleading than useful. It is rather a non- cellular than a unicellular being; it has not entered upon the cellular line of evolution; it is a complex organism with much division of labour, and no Metazoan cell can be said to be on a par with it. Similarly, it is a suggestion of a fallacy in calling an ovum a single cell; for it has a complexity beyond imagining, it is a highly endowed implicit organism, (b) It is no longer easy to be satisfied with the oft-repeated comparison of a multicellular organism to a colony or regiment of cells. It has become clearer that cellular structure is largely a segregating device for the better working of that division of labour which the intricacy of vital processes demands, (c) If we take as a good example the recent Introduction to' Cytology (1920) by the late Prof. Doncaster, we find that our'picture of the animal cell has become extraordi-

narily complex. In the cell-substance or cytoplasm there are in many cases definitely formed granules or rods (mitochondria) which sometimes have to do with the formation of particular protoplasmic products; there are very frequently strands or rods of the " Golgi apparatus," the significance of which is very obscure; there are also " chromidia " which sometimes appear like migrants from the nucleus attempting to colonize the cyto- plasm. In the centre of the cytoplasm floats the nucleus, mi- crocosm within microcosm. It has its differentially permeable membrane; its chromosomes, usually definite in number for each species; its nucleolus, which may be a karyosome of chro- matin or a plastosome of plastin; and the karyolymph, bathing both chromosomes and nucleoli. Then there are the centrosomes, which play an important part in organizing the process of nu- clear division. But this is not nearly all, for each chromosome is like a necklace of beads threaded on a transparent ribbon of linin, and these beads or microsomes are probably the biological units of the lowest visible grade. In curiously indirect ways it seems possible (Morgan, 1919) to make a sort of map of the chromosomes of an egg-cell, and to say, for instance, that such and such hereditary factors of the fruit-fly Drosophila are located in the upper third of the second chromosome. In some cases it is possible to tell from visible peculiarities in the chromosomes of a fertilized egg-cell whether it would have developed into a male or into a female. These are but illustrations of the increasing precision, (d) In another direction, however, modern work has led to' simplification, namely as regards cytoplasmic structure. The 19th-century histologists accepted with little question the view that the reticular, fibrillar, or other fine structure seen under high power in fixed and stained cells corresponded to a genuine architectural complexity in the living cell. But the work of Hardy (1899) and Fischer (1899) showed that the alleged structure is mainly of the nature of artefact, and that different structure is revealed according to the histological methods used. The use of the ultra-microscope has confirmed the con- clusion that protoplasm in a living condition is a structureless fluid with particles and droplets in a freely movable state a colloidal system in short. There must be arrangements which permit of the simultaneous occurrence of very different chemical processes in adjacent parts of the cell, but this is not of the nature of a visible cytoplasmic architecture, and it may be of the nature of a temporary gelation (see Bayliss, 1915, chap. i.). (e) Just as the early microscopists described and figured many structures under magnification without making them more significant or intelligible, so in the immense library of zoological histology there is a prodigious amount of meticulous description that is not very illuminating. Too much of it has been a registra- tion of artefacts; and it may be fairly noted as characteristic of recent years that mere micrography is finding fewer devotees. The achievements of differential staining are giving place to a reasonable biochemical microscopy. For it is plain that the analytical study of minute structure does not justify itself apart from demonstrating specificity and a succession of plasmic phases unless it throws light on what happens in the cell- laboratory the oxidations and reductions, the hydrations and condensations, the synthetic and analytic processes which con- stitute vital metabolism. These chemical reactions take place with extraordinary speed, which is conditioned by the activity of enzymes, and also with not less extraordinary orderliness, which is conditioned by some localizing (as it were insulating) arrangements of the colloidal system of the cell. But an analogy probably more fitting than that of a laboratory is that of a factory, for it seems clear that cytoplasm, nucleus, centrosomes, mito- chondria and so forth are working together, and potent in their inter-relations.

Thus, to take an obvious case, there appears to be, as Richard Hertwig and others have emphasized, a definite volumetric ratio between the nucleoplasm and the cytoplasm of the cell. Interesting results have also rewarded the inquiry into the changes in this relation which occur in regenerative processes, in the growth of tumours, in the segmentation of the ovum, when a microbe enters a cell, and disturbs its equilibrium.