HI7TCHINSON'S NATURE LIBRARY.*
THESE two volumes in a new series of books of popular natural history are full of promise. If the rest of the Nature Library keeps up the same standard of text and plates its success ought to be certain. But since books of natural history quickly become antiquated we observe with regret that neither volume bears a date on the title-page. 'Unless it be to prevent the purchaser discovering that the book he is buying was published some time ago, the object of this omission is not apparent.
Strangely enough, Mr. W. P. Pycraft and Dr. Chalmers Mitchell (whose book we recently reviewed) have almost simultaneously hit upon the same theme. On the 'open- ing page of his Infancy of Animals Mr. Pycraft declares that one may search in -vain for any systematic attempt to depict the earlier stages in the life-his tories ,of animals. And this was true when it was written. Which was first in the field is of no moment ; for both books are excellent, and good books, in which the latest views of scientific _zoology are made accessible to the layman, are always to be welcomed. The study of animals in their infantile state has only recently been seriously undertaken. It is, in fact, a continuation of the study of embryology, which has been so fertile in suggesting new theories of descent and relationship. There is, as Mr. Pycraft truly points out, no hard-and-fast line to be drawn between embryonic and post-embryonic stages, between larval and post-larval life. Mr. Pyoraft treats here of young mammals, birds, reptiles, amphibians and fishes. Invertebrates are accorded two short chapters. Birds, being a specially favourite subject of the author, receive a good deal of space. Indeed, nothing can better throw light on the origin of birds than the strange early life-history of the hoatzin. So in the -tadpole we trace the transformation from fish to reptile ; and birds, as every- one knows, are but glorified reptiles. Mr. Pycraft's enormous knowledge of comparative osteology enables him to teach the lesson of evolution with great effect. In writing of the young animal he is able to make comparisons .and to trace descent, modification, and relationship of form. Sometimes his own familiarity with the jargon of zoologists leads him to use technical words which are unnecessary, and which alarm the lay reader. But the book is not a stiff one. It is natural, perhaps, that Mr. Pycraft, of the British Museum, should treat his subject a little more dryly than Dr. Chalmers Mitchell, of Regent's Park. He makeshis matter, _however, interesting, particularly where he points out the numberless records of the past which close study reveals in young animals. There are numerous good illustrations of a very instructive kind. In the plate facing p. 140 the skull described as "ostrich" appears to be that of an emu.
Everyone knows that the young sparrow is naked, helpless, and blind when it comes from the egg, whilst a young duck is clothed in down and able to run, swim, and feed itself. Ornithologists, however, have not until lately set themselves to explain the differences between the so-called " nidicolous " and "nidifugous " types of young. In fact, no hard-and. fast line can be drawn between the precocious and helpless • (1) The Infancy of Animala. By W. P. Pyczaft, ttc. With 64 plates on art paper and numerous illustrations in the text. London Hutchinson and Cu. [es. net.}—(2) Messinates : a tRook of Strange Cerapanionships in _Nature. Ry Edward Step, T.L.S. With 65 illustrations on art per from original vhalographs and Arawinge. Ramo publishers and price.
types. We find all stages of development, and, what is more remarkable, closely allied birds may in their nestling stages be exceedingly different or be exactly similar. Probably the precocious type is the more primitive and ancient, since, as we have already once pointed out, birds are but glorified and highly developed reptiles; and young reptiles are always active at their birth. The young South American hoatzin, to which we referred above, more closely resembles the oldest known fossil bird than does any other living species. It climbs from its nest into the branches, using its feet and the claws with which its wing digits are furnished. Curiously enough, the development of the quills in the wings of young and precocious game birds, though the matter is too complicated to explain in detail here, shows traces of the days when a free finger-tip to the wing served as a climbing organ. Mr. Pycraft thinks that all young birds were originally arboreal and active. Then natural selection acted along two divergent paths. In one type we have activity on the ground end protective coloration. In the other type of bird, extreme helplessness is produced by reducing the amount of food yolk in the egg.
In young mammals, to deal only with one striking side of the matter, we have to explain some difficult problems of colouring. An immense number of young mammals are striped or spotted when their parents are plainly coloured. This may represent the coloration of ancestors or it may be protective or possibly both. It is difficult to believe that the spots or stripes are produced by existing factors of selection. Yet if we believe striping to be ancestral it is surprising to find among the primitive marsupials hardly any striped animals. Spots in the young are but stripes broken up. This is clearly seen in many animals, as, for instance, the young musk deer. But why the adult musk deer should be self-coloured and the adult axis deer -spotted is a mystery wbich neither Mr. Pycraft nor anyone else has yet explained. In the reindeer, spots have vanished both from adult and young. No one seems to have had the curiosity to investigate whether the reindeer before birth has any traces of spots. Another mystery is the harnessed antelope, in which the adult has not only spots but longitudinal and transverse stripes. To say of Mr. Pycraft that he states many facts without explaining their bearing and propounds many problems with solving them is not to make any unfavour- able criticism. The study of the infancy of animals is itself in its infancy, and it is unwise to lay down hasty generalizations.
In his chapter on the "Infancy of Fishes," Mr. Pycraft touches upon several strange phenomena of animal life which form the subject of the second book in this review, Messmates, by Mr. Edward Step. Probably Mr. Step has chosen that title because the words Symbiosis, Mutualism, and Commensalism are somewhat alarming. It used to be thought, when two animals of different species were found in close association, that one was a parasite on the other. Now it has been observed that in a vast number of such cases the partnership, which may be between animals or plants, or a plant and an animal, is to the advantage of both. We are dealing then with Symbiosis and a phase of life which offers most interesting matter for study to the evolutionary biologist.
Nearly everyone knows that the familiar and almost ubiquitous lichens are not a distinct form of plant life, but a partnership between a fungus destitute of chlorophyll and a green-celled alga. Here is a partnership of mutual advantage. So too in a polyp, Hydra viridis, the colour is due to one-celled green plants. In a fiat-worm found on seaweed the animal languishes unless its body gets infected with algal cells. Most people have heard also of the minute bacteria which make nodules full of nitrates on the roots of leguminous plants which the plant can use as food. Virgil knew that beans and similar plants of the pea family enriched the soil, but, of course, was ignorant of the reason. These are all cases of the simplest but most complete form of symbiosis. The flat-worm Convoluta cannot live long without its algal partner.
Among the higher animals there are cases of a similar nature. Some are difficult to explain unless there is mutual advantage. Heroclotns tells the story of the plover which picks fragments of food and leeches from the crocodile's; mouth and gives a danger signal when the reptile sleeps. There seems to be some truth in the ancient writer's tale. The buffalo-bird warns the buffalo and feeds on maggots or ticks which infest the beast's bide. There are fishes, too, which habitually set up partnerships with sea-cucumbers and jelly- fish. The advantage to the fish is plain, but what benefit the other partner gets is not so obvious. The most remarkable and extraordinary case is perhaps the relation that is established between the fresh-water mussel and the bitterling, a small carplike fish of Central Europe. Each becomes foster-parent to the offspring of the other. The female fish introduces the tip of a long ovipositor into the open shell of the mollusc and lays her eggs between the gill-folds. The infant fish are hatched and remain between the gillplates a month or more until they are grown big enough to take care of themselves. At the same moment that the bitterling lays her eggs the mussel discharges a shower of her embryo offspring or fry on to the fish. The young mussels by hooks on their toothed shells attach themselves to the bitterling and remain embedded in its skin until they have gone through certain transformations and fall out to the bottom of the stream to found fresh colonies.
Mr. Step has the happy power of writing a clear and simple account of this strange side of animal and plant life. He is primarily a botanist and does not neglect the plant side of these symbiotic unions. Ants and the numbers of insects which share their nests afford many instances of a pseudo- partnership. Over nine hundred species of beetle have now been detected as companions of ants. Many are tolerated and some actively encouraged by the ants. Some, like the aphis, appear to be milked of a sweet liquid. Others possibly act as scavengers. Some perhaps are pets and of no more use to the ants than white mice or guinea pigs are to mankind.
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