outlines foreshadow tolerably well the actual details of the finished work. And what is true of the relations between reptiles and birds, or of those between the various races of crocodiles-which, it is important to note, living and extinct, are bound together in a series almost as graduated and complete as are the horses and their progenitors-what is true of the connecting links betwixt quadrupeds that to-day appear distinct and separate, must by every consideration, alike of logic and common sense, be held to apply with equal force to the entire world of animal and plant life. There is no law of evolution for one group, and of special creation for another. There can be logically postulated no evolution for the lower races, and some process of "creation" for the higher forms of animal life or for man himself. Uniformity and sequence exist wholly, or not at all. "If one series of species," says Huxley, "has come into existence by the operation of natural causes, it seems folly to deny that all may have arisen in the same way." The unbiassed mind, contemplating the varied phases of living nature, will stand in no dread of any conclusions respecting the order of this universe, to which evolution may lead; for, after all, evolution, in tracing out the ways of nature, is but the handmaid of truth, and it is with the truth as it is in nature, that the earnest mind will most desire to close.

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IX.

THE EVIDENCE FROM DEVELOPMENT.

I. THE EARLIER STAGES IN THE LIFE-HISTORY OF ANIMALS.

AMONG the many features which mark the varied universe of life, none are more universally recognised, or more typical of the living world, than those which herald the production of a new being, and which usher a new form upon the stage of existence. From the shapeless mass of protoplasm that crawls over the waterweed as a microscopic speck, upwards to man himself, the varied processes of development are laid down in orderly sequence and along lines of special kind. Every living being, animal or plant— animalcule and whale, the humble lichen and the giant sequoia alike -passes through a definite series of changes before attaining the form and likeness of the parent which gave it birth. In virtue of such changes it assumes that parental form. These changes, occurring in orderly array, mark its pathway from shapelessness and physiological nonentity to the characteristic form of its race. It is development which moulds

The baby figure of the giant mass,

and from the minute beginnings of life evolves the highest of earth's denizens, or directs the production of the teeming swarms of animalcules that people the stagnant drop, and pass an existence none the less interesting or important because often all unknown to the larger and higher world without. It is this same process of development which, as one phase of living action, draws the sharpest and clearest of boundary lines between the world of life and that of non-living matter. Growth and increase are truly represented in the inorganic world; but these processes are different in kind from the actions which stamp the development of the animal or plant. The birth of a crystal, although regulated by definite laws, is, after all, a matter of outside regulation alone, and one in which the crystal itself is but a passive agent. New particles are added to the outside surfaces of the old and already formed particles; and crystal and stalactite thus grow mechanically and by accretion, but without active participation in the work destined to mould and form their substance.

Very different are the forces and laws which regulate the production of the living form. Here the changes of form and the

building of the frame are marked out in plain and definite pathways by laws essentially independent of external conditions. True, the development of the living form may be retarded by cold or favoured by warmth, but these conditions leave unaffected the course and direction in which it is destined to pass towards the form and belongings of the parent which gave it birth. Stamped ineffaceably on the pages of its life-history, the way of the animal or plant towards maturity is written for it, not by it. Internal forces and hidden but all-powerful laws of life direct its progress, and ultimately evolve the perfect being from the shapeless germ, in which its past as derived from its parents, and its future as depending in some degree at least upon itself, meet in strange and incom prehensible union. The development of a living being may be further shown to be merely a part of the wondrous cycle in which life appears to direct its possessors. From the egg or germ, development leads us to the perfect being. Next in order we consider its adult or perfected history; and in due time we may discover the adult existence to merge into that of the immature state in the production of germs, in the development of which its own life-history will be duly repeated. The period of adult life in this view merely intervenes betwixt one development and another, and serves to connect those ever-recurring stages in the life-history of the race which it is the province of development to chronicle and record.

As a necessary item in the perfect understanding of animal and plant history, it may readily be understood how important a place development occupies in modern biology. Nor is the interest of the study excelled by its importance. The mystery of life itself might well be thought by the older physiologists to resolve itself into an understanding of the fashion in which Nature moulded and formed her varied offspring. The manner of development might be almost expected to explain the mystery of being; but the problem of life is left as insoluble as before, after the course of development in even the lowest grades of existence has been traced. The history of development but environs the puzzles connected with life and its nature. It leads us to the beginnings of life, it is true, but it leaves these beginnings unaccounted for, and as mysterious as before. It explains how this tissue or that, this organ or that, is fashioned and formed; and as we watch the formless substance giving birth to the formed, the indefinite evolving the defined, we might well be tempted to think that the "why" of nature was explained by the "how." Yet the springs of life and vital action remain hidden as of yore, and the exact origin of life is a mystery as insoluble as when the thoughts of men were first directed to its elucidation.

Apart, however, from the admission that the study of development has not brought us nearer to the solution of the question, "What is

life?" the investigation of the life-histories of animals and plants is fraught with high importance in another sense and in other aspects of the scientific interpretation of nature. The early observers hardly imagined that, in their researches into the formation of the chick, they were laying the foundation of a study which in future days would be destined to aid man's comprehension of his own origin and that of all other living beings. Aristotle's observations upon the developing chick, and his bestowal of the name punctum saliens, or beating point," upon the first beginnings of the heart in the embryo bird, were in truth fraught with an importance to succeeding generations which that philosopher could barely have realised had he possessed any prophetic foresight whatever. And no less would Harvey himself have been astonished had he beheld the results to which the pursuit of his favourite study has led in these latter days. It was that great philosopher himself who first maintained that the chick was developed, not from the white of the egg, but from a minute speck or scar on the surface of the yolk, known as the blastoderm or cicatricula. In felicitous terms Harvey enunciates his opinion that the "Medici," or disciples of Galen and Hippocrates, were in error when they supposed that such important structures as brain, heart, and liver were first produced, simultaneously, as minute sacs or vesicles; and he disagrees with Aristotle, in respect that the latter had maintained the punctum saliens [or punctum sanguineum], or heart, as the chief agent in forming the structures of the new being. Harvey ascribed to the blood itself the formative power in developing the chick. With Aristotle, however, Harvey is in perfect agreement in believing that the chick is formed, not by the sudden formation of new parts outside the already formed organs, nor by the growth of a miniature and perfectly formed embryo into the larger chick, but by the gradual development and elaboration of uniform and like matter into the new and varied parts and organs of the bird.

Such were Harvey's views regarding the nature of development. Of the supreme interest exhibited by the discoverer of the circulation in this study, no better proof could be cited than his own words when he maintains "that it is most apparent that, in the generation of the chicken out of the egge, all things are set up and formed with a most singular providence, Divine wisdom, and an admirable and incomprehensible artifice." Harvey's doctrine of development received the name of Epigenesis, which the physiologist himself defines, in his forty-first "Exercitation," as "the additament of parts budding one out of another." Contrasted with this opinion, is that of such physiologists as Malpighi and Leibnitz. They held that the body of the chick could be traced in the egg before the first rudiment of the heart appeared; and that from the first formation of the egg, and prior to incubation, the young bird was to be found

perfectly formed therein. Thus, by Malpighi's view, the process of development was merely one of the expansion, unfolding, and enlargement of parts already formed; and this idea became known as that of Metamorphosis, in contradistinction to Harvey's theory of "Epigenesis." So, also, Bonnet maintained the existence of a miniature chick in the egg from the first moment of its formation. Subsequent growth and nutrition merely expand the elements and parts of this germ into those of the adult; and thus Bonnet declares the process of development to be merely one of "Evolution." Thus the doctrine of "Epigenesis," as enunciated by Harvey, becomes opposed to that of "Evolution," as maintained by Bonnet and Haller-the development of new parts and structures from a structureless substance, as distinguished from the mere enlargement and unfolding of the miniature but already formed elements of the frame.

But when Bonnet, in 1762, in his work entitled "Considérations sur les Corps organisés," was elaborating his theory of "Evolution" and less rational views on "Emboîtement"-a theory holding that each germ is the receptacle of the germs of all future beings of its race Caspar Friederich Wolff had already lent his aid towards placing the Harveian views on a secure and stable basis. Wolff showed that the scar on the hen's egg consisted of particles amidst which no rudiment of an embryo chick could be traced. He further demonstrated the changes whereby the chick was built up from these cells, and showed the process of development to be truly one wherein new parts were formed in succession, and added to the already formed organs. Succeeding Wolff came Pander, who filled in the outlines his predecessor had so well sketched out by detailing the earlier stages and processes seen in the formation of the young bird. From Pander came the name blastoderm, given then, as now, to the substance or formative material resulting from early changes in the "egg-scar," and from which material all the parts of the young animal are formed. This observer also cleared the way for his successors by pointing out the presence of the three layers into which the blastoderm divides; each layer bearing an important share in the formation of the tissues of the developing being. To Pander came in due time a worthy successor, who may be said to have laid the solid foundations of the study of development as prosecuted in modern times. This was Von Baer, whose labours each physiologist and naturalist of to-day must hold in grateful remembrance. He it was who, besides perfecting the details already to hand, discerned the important fact that the highest animals are developed from eggs or germs resembling in essential nature those of the lowest. But perhaps the greatest triumph of discovery and research as represented by Von Baer's labours resulted in the enunciation of his "law of deve