SCHOOL IN THE OPEN.

Star (Christchurch), Issue 18900, 26 October 1929, Page 24 (Supplement)

SCHOOL IN THE OPEN.

STUDIES IN THE GREAT OUT-OF-DOORS.

(By J. J. S. Cornes, 8.A., B.Sc.) The “ Star ” has arranged with Mr J. J. S. Cornes, 8.A., B.Sc., to write a series of illustrated articles which will give teachers and others a fuller appreciation of the Great Out-of-Doors They will deal with various aspects of plant and animal life, as well as with inanimate nature. Questions and material for identification will be welcomed.

EMBRYOLOGY. CCCXII. BIOLOGY. XVII. We have seen that the resemblances that exist between animals included in any of the big classificatory groups, and similarly with plants, constitute a strong reason for believing that they are descended from a common ancestor. It may. be asked, “Is there any evidence that the big groups themselves are similarly related ? Any evidence that the Protozoa and the two-cell-layered Coelenterates and the three-cell-layered Coelomates are sprung from a common stock, that even animals and plants are blood relations by descent?” There is, but we will now confine our attention almost entirely to animals, setting out evidence for the truth- of the Evolution Theory from (1) development of the fertilised ovum (Embryology), and (2) from extinct animals (Palaeontology). Embryology. If a series of examples of any three-cell-layered animal be examined at even younger and younger stages of development, there is at length reached a point at which the embryo does not contain three chief layers of cells, but two only; and of these two the inner (endoderm) is the lining of the incipient digestive tube. That is, the threelayered condition of Coelomates arises from a two-layered condition similar in essentials to that of Coelenterates. The Gastrula. This stage, through which every Coelomate passes, is known as the Gastrula. Its form is often much modrned by the presence of food-yolk, but, when due allowance is made for this, it is always recognisable. The conclusion that all Coelomates, from ■worm to man, were evolved from twolayered creatures similar to existing Coelomates is irresistible; no other satisfies our reason.

Pedigree of Metazoa. Now, if our imaginary series of examples be carried back to yet younger stages than the gastrula, a stage is reached in which the embryo does not possess two layers of g&Us. nor any trace of the beginnings of a digestive tube, but consists of a hollow sphere of cells arranged in one layer only, and the cells all fundamentally alike.

If we carry our series yet further back, we find that the ball of cells is produced by the repeated divisions of the single cell, the fertilised ovum; at the outset, then, every Metazoan is a single cell like a Protozoan. This cell is a complete animal potentially; it possesses in a latent condition all those properties that subsequently are manifested and brought into action by the various tissues of cells to which it gives rise by its segmentation. For these remarkable facts the only explanation is that the pedigrees of Metazoans goes right back to Protozoan ancestors.

Pedigree of Plants. But plants also begin existence as a single cell, the fertilised ovum. So we are faced with the fact that all living things begin the same way, and are justified in the conclusion that the plants and animals which to-day people the earth have all, in the course of long ages, been evolved from unicellular ancestors —though it is not conceivable that life first manifested itself in a structure so complicated as even a single cell is., - The Recapitulation Theory. If we admit that in adult animals! iniiionnmninniniiiiniiiiinniniiniinnnnniiniinnniiniiiiiiiiiinniinnniiiiinni

structural resemblances justify the belief that there is a blood-relation be-1 tween them, it is impossible to reject the same belief regarding resemblances at earlier stages in their life-histories. Thus, embryology has proved of immense value in endeavouring to trace the relation between different groups of animals. The theory underlying all such research is known as the Recapitulation Theory, which, briefly, amounts to this: that every animal in the course of its individual development from the ovum to the adult, passes through the stages through which the group of which it is a member passed in the course of its evolution. In a few weeks or months each animal, as it develops, presents a very condensed, and often much-modified, epitome of the evolutionary history of its group during untold millions of years: personal history recapitulates race history.

Instanced by the Frog. From the wealth of instances that might be quoted we take that of the common frog. It begins life as a single cell, a yolk-laden ovum, in a mass of spawn. The ovum segments, giving rise to a spherical mass of cells, comparable to a compound Protozoan. By certain changes and growth the spherical mass becomes twolayered, and the gastrula stage is reached, much modified by the inertia of its yolk legacy, yet quite comparable to a Coelenterate. Then at one bound, so to speak, the embryo reaches a stage at which it is clearly a vertebrate animal. There is no trace of worm, insect, or mollusc in the pedigree, nor are there any other valid reasons for thinking that they were so. In a few weeks the tadpole stage is reached; and now, in the possession of gills, and in the arrangement of the chief blood-vessels, and in some minor details, the creature is virtually a fish—that is, it has reached the group, Fish, from which, apparently, its own group, Amphibia, was evolved. ! Eventually it loses its gills and other imiiiinnnnumiminiinininininnnununnmir.miiinnmitiinimnimnmimfiP

piscine features, such as the sculling tail, breathes by lungs, and develops legs and leaves the water as a little frog—an Amphibian. It has climbed its own genealogical tree. (To be continued next Saturday.)