Imágenes de páginas
PDF
EPUB

Yet the oceanic waters to-day swarm with swimming life, and in all probability did so then. This life, as now existing, contains many high as well as numerous low forms. Then it must have consisted of low forms only. The wealth of existing minor sea life, as observed by the unassisted eye and revealed by the microscope, is simply boundless.

Small jelly fish are met with in vast armies, hundreds of miles in extent, and descending to many feet in depth. Pteropods, both the naked and the shelled forms, occur in prodigious multitudes. The minute copepod crustaceans are found in countless swarms, and, though consumed in myriads daily by herring and other fish, by medusæ, siphonophora and other invertebrates, and even by the whale, they are so productive that their numbers seem undiminished, being found over vast areas of surface and extending through more than a mile in vertical depth. Below these again are hosts of microscopic larve and minute animals, and still lower are countless swarms of protozoa, such as radiolarians, globigerinæ, etc.

Here, then, are innumerable swarms of swimming and floating forms, in most part carnivorous, but necessarily requiring a vegetable basis of nutriment. The foundation food supply for such a mighty host must be enormous in quantity. The visible plant life of the ocean, the alge which grow on the bottom, would not sustain a tithe of such an army. The microscope must again be brought into requisition, and this useful instrument reveals to us an extraordinary profusion of unicellular plants—diatoms, coccospheres, trichodesmiums, and a few other types—which extend from the surface to the lowest level of light penetration, and are so extraordinarily numerous and prolific as to supply food for all the oceanic host. These, and the protozoa which feed upon them, form the basic food supply for the countless myriads of living forms which compose the fauna of modern seas.

Yet, were the conditions of the ocean as they exist to-day to be sought for by some far future geologic delver into the mysteries of the rocks, almost nothing of this profusion of life would be revealed, discovery being nearly or entirely confined to such forms as possess hard skeletons, internal or external, of which most of these forms are destitute. The same was probably the case with the period which we now have under review, and of whose life we find few forms except those which habitually dwelt upon the bottom. The ocean may have been as full of life then as it is to-day, many of the swimmers of that period, perhaps, representing the ancestral lines out of which the bottom dwellers had evolved, and which are still in a measure preserved for us in modern embryos. These primeval forms may have been even less suitable for fossilization than their counterparts of to-day. The diatoms, the radiolarians, and other minute existing forms have silicious shells capable of preservation. It is quite possible that the early protozoa and protophytes had no such skeletal parts, and that when they died all trace of them departed.

How far back, then, from the earliest age of fossils must we place the actual date of the origin of life? Ages perhapsepochs—a period as remote from the Cambrian in one direction as we are in the opposite. It may have taken as long, or longer, to develop the trilobite as it since has taken to develop

During the whole of the immensely long period in which the miles of earlier strata were being deposited, the ocean may have been the seat of an abundant life of the lowest type, and this a very slowly evolving one, the conditions being such that competition and the struggle for existence were not strongly active.

Of the forms of life now existing, the most abundant and the lowest in organization known to us are the bacteria or microbes—omnivorous life specks, feeding alike on animals and plants, and fairly assignable to neither. Possibly life had its origin in forms like these, or in still lower stages of protoplasmic activity, and from this condition developed, after an interminable period, into the simple oceanic protozoa and protophytes typified by the radiolorians and the diatoms, the lowest forms having characters common to both animals and plants, while their descendants divided definitely into plants and animals.

The period here referred to, and that subsequently consumed in the development of the trilobite and its companion forms,

man.

must have been of very great duration; for the conditions were such as to make evolution a slow process. The habitat of these primeval life forms, the oceanic waters, was of the greatest uniformity, even probably in temperature, and possessed no condition likely to provoke rapid variation. There was abundant space and probably abundant food, particularly in view of the minuteness and slight nutritive demands of these early animals, and the struggle for existence could not have been active. Though there were millions devoured hourly, there were trillions provided for the feast, so that no great tendency towards the preservation of favorable variations would have existed.

Yet, though the influences which favor evolution were not very actively present, they could not have been quite absent. The innate tendency to vary which all living forms possess now must have existed then, and the advantage possessed by the more highly over the more lowly organized forms could not have been quite wanting. Consequently, development of varying life forms must have gone on at some rate, and animals must in time have appeared much higher in organization than the simple forms from which they emerged.

And the variations which took place were radical in character. Variation in the higher recent types of life does not penetrate deeply. After ages of change a vertebrate is a vertebrate still. Millions of years of change do not convert a cat into something radically distinct from a cat. But in the primitive period the changes were more profound. Variation went down to the foundation plan of those simple forms and converted them at once into something else. A degree of variation which now would modify the form of a fish's fin may then have converted a monad into a new type of animal. Thus primitive evolution, working on forms destitute of any definite organization, may readily have brought into existence a number of highly different types of life.

As the microbe, for instance, may through long variation have given rise to the two organic kingdoms of animals and plants, so the amoeba or other low animal form may have varied into the subkingdoms of mollusca, echinodermata, coelenterata, etc., or rather into simple swimming forms each of which was the progenitor of one of these great branches of the tree of life.

We are here in a realm of the unknown, through which we are forced to make our way slowly and uncertainly by aid of the clues of embryology, microscopic life conditions, principles of variation and development, and the known conditions of pelagic life. We can only surmise that, as the result of a long era of evolution, the simple primary forms gave rise to a considerable variety of diverse animals, still comparatively minute in size and simple in organization, swimming by means of cilia, and typified to-day by the swimming embryos of invertebrate animals.

As yet—if our hypothesis is well founded-no life existed upon the bottom of the seas, and the swimming forms were destitute of any hard parts capable of fossilization. But why did not some of these forms very early make their way to the bottom and begin life under the new conditions of contact with solid substance ? And yet why should they have sought the bottom ? Their food supply lay on or near the surface, the bottom of the shallow waters may have been unsuitable through the deposition of soft sediment, and the bottom of the deeper waters very sparse in food. And, more important still, they were quite unadapted to life on the bottom, and needed a radical transformation before they could survive under such conditions. If we look at the remarkable change which the swimming embryo of a star-fish or sea-urchin, for example, goes through before any resemblance to the mature form appears, we may gain some idea of the long series of variations which the primitive ciliated swimmers must have passed through to convert them into crawling or stationary bottomdwelling forms. Great as was the period needed to produce these type forms of life, another extended period must have been necessary to convert them into well adapted habitants of the solid floor of the seas.

(To be Continued.)

BIRDS OF NEW GUINEA (FLY CATCHERS AND

OTHERS).

By G. S. MEAD.

Among the many kinds of Flycatchers (Muscicapidæ) inhabiting the Papuan Islands, while there is dissimilarity in so large a number of species, yet there are not those striking differences amounting almost to contrasts which characterize birds of greater size. Many species have been unnoticed by travellers and other writers; many exist only in cabinets and collections, labelled and ticketed, or at most given a few lines of technical summarization in catalogues. With the rank and file of birds anything more than this is impossible. Sometimes a particularly attractive specimen of Malurus or Rhipidura or Pratincola calls attention to itself, or mere accident brings an individual to the notice of the explorer or student.

Thus Mr. Wallace notes pointedly " the abnormal red and black flycatcher,” Peltops blainvillii, so named by Lesson and Garnot many years since. It is a sprightly, highly colored bird with the predominant hues strongly contrasted and still further accentuated by spots of white on the head and beneath the wings. In flight this active little flycatcher presents in turn these conspicuous markings with striking effect. The red tint is a bright crimson spread over the lower back and tail coverts. The main color is a steely-green black covering with greater or less intensity the seven inches of total length. The genus is represented by this species only.

The same notable expedition to South Eastern New Guinea that secured the two beautiful prizes Chemophilus macgregorii and Amblyornis musgravianus, discovered also a new species of flycatcher, viz., Rhipidura auricularis. It is described as hav.

upper surface smoky gray; head brownish black ; tail the same above and below; bill dark brown; legs black." The head is marked by black and white stripes, found upon the wings as well. Upon the chin, throat and breast similar

ing the “

« AnteriorContinuar »