ence is one due to the environment of the individual. Those in the sea find adequate salts from which to develop their coats of mail. Those in fresh water do not find this, while those in river mouths and other brackish situations develop armature in intermediate degrees. In the genus Eucalia, a stickleback confined to fresh waters of the Middle Western States, plates are never developed.

The Loch Leven trout, Salmo levenensis, is distinguished from the brook trout of England, Salmo eriox (fario), in its native waters by certain obvious characters. These disappear when the eggs are planted in brooks in England or in California, and the species develops as the common English brook trout. But it is conceivable that the obvious or ontogenetic traits of the Loch Leven trout are not the real or phylogenetic distinctions, and that the latter, more subtle, engendered through individual variation, inheritance, selection, and isolation, really exist, although they have escaped the attention of ichthyologists.

After the Loch Leven trout was planted in the Yosemite Park in 1896, it remained for nine years unnoticed. In 1905 individuals sent to Stanford University were, so far as could be seen, exactly like English brook trout. But it is conceivable that differences in food and water have caused slight ontogenetic distinctions. It is certain that in isolation from all parent stocks they will in time develop larger differences which, after many thousand generations, will be specific or subspecific. At present, these trout are quite unlike the native rainbow trout (Salmo irideus gilberti) of the Yosemite. The ontogenetic characters will perhaps approach those of the latter, but the phylogenetic movement may be in quite another direction.

Another ontogenetic species is the little char or trout (Salvelinus tudes Cope) from Unalaska. In Captain's Harbor, Unalaska, the Dolly Varden trout, Salvelinus malma, swarms in myriads, in fresh and salt water alike, reaching in the sea a weight of from six to twelve pounds. A little open brook, which drops into the harbor by an impassable waterfall, contains also an abundance of Dolly Varden trout, mature at six inches and weighing but a few ounces. This is Salvelinus tudes. In the harbor the trout are gray with lighter gray spots, and fins scarcely rosy. In the brook, the trout are steel blue, with crimson spots and orange fins, striped with white and black. In all visible

phylogenetic characters, the two forms of trout are one species. We have reason to believe that fry from the bay would grow up as dwarfs in the brook, and that the fry from the brook would be gray giants if developed in the sea.

But it is also supposable that in the complete isolation of the brook fishes, with free interbreeding, there would be some sort of phylogenetic bond. There may be a genuine subspecies, tudes, characterized not by small size, slender form, and bright colors, but by other traits, which no one has found because no one has looked deeply enough.

In no group of vertebrates are the life characters more plastic than among the trout. The birds have traits far more definitely fixed. Yet differences in external conditions must produce certain results. We should not venture to suggest that the dusky woodpeckers or chickadees of the rainy forests of the northeast and northwest are purely ontogenetic species or that they should be erased from the systematic lists. But it will be a great advance in ornithology when we know what they really are and when we understand the real nature of the small-bodied, largebilled, southern races of other species of birds. It would be worth while to know if these are really ontogenetic purely, or if they are phylogenetic through "progressive heredity," the inheritance of acquired characters, such as are produced by the direct effects of climate or as the reaction from climatic influences. Or again may there be a real phylogenetic bond. through geographical segregation, its evidences obscured by the more conspicuous traits induced by like experiences? Or are there other influences still more subtle involved in the formation of isohumic or isothermic subspecies?

To sum up, there is no convincing evidence that the direct influence of environment is a factor in the separation of species, except as its results may be acted upon by natural selection. We have no proof to show that the environment of one generation determines the heredity of the next-and yet perhaps most naturalists feel that the effects of extrinsic influences work their way into the species, although a mechanism by which this might be accomplished is as yet unknown to us.

CHAPTER XII

GENERATION, SEX AND ONTOGENY

"Unter jedem Grab liegt eine Weltgeschichte" (German proverb).

EACH animal, each plant, must have its individual beginning, its "creation," and its individual development from this beginning to full grown, completely developed condition. For no organism is born fully developed. Even the simplest organisms, the one-celled kinds, whose "creation" is accomplished simply by the splitting in two of a previously existing individual of their kind, are not produced full-fledged. They have at least to increase from half size to full size, that is, to grow, and there are very few if any of them that do not have to effect changes in their body structure during this period of growth; that is, they have to undergo some development. The beginning, then, is always from a previously existing organism-but how could it always have been?-and between this beginning and the normal mature or full-fledged creature there has always to be some development. The beginning is called generation; the development, ontogeny.

We are all so familiar with the fact that a kitten comes into the world only through being born as the offspring of parents of its kind, that we shall likely not appreciate at first the full significance of the statement that all life comes from life; that all organisms are produced by other organisms. Nor shall we at first appreciate the importance of the statement. This is a generalization of modern times. It has always been easy to see that cats and horses and chickens and the other animals we familiarly know give birth to young or new animals of their own kind; or, put conversely, that young or new cats and horses and chickens come into existence only as the offspring of parents of their kind. And in these latter days of microscopes and

mechanical aids to observation it is also easy to see that the smaller animals, the microscopic organisms, come into existence only as they are produced by the division of other similar animals, which we may call their parents. But in the days of the earlier naturalists the life of the microscopic organisms, and even that of many of the larger but unfamiliar animals, was shrouded in mystery. And what seem to us ridiculous beliefs were held regarding the origin of new individuals.

The ancients believed that many animals were spontaneously generated. The early naturalists thought that flies arose by spontaneous generation from the decaying matter of dead animals; from a dead horse come myriads of maggots which change into flesh flies. Frogs and many insects were thought to be generated spontaneously from mud. Eels were thought to arise from the slime rubbed from the skin of fishes. Aristotle, the Greek philosopher, who was the greatest of the ancient naturalists, expresses these beliefs in his books. It was not until the middle of the seventeenth century-Aristotle lived three hundred and fifty years before the Christian era— that these beliefs were attacked and began to be given up. In the beginning of the seventeenth century, William Harvey, an English naturalist, declared that every animal comes from an egg, but he said that the egg might "proceed from parents or arise spontaneously or out of putrefaction." In the middle of the same century Redi proved that the maggots in decaying meat which produce the flesh flies develop from eggs leid on the meat by flies of the same kind. Other zoologists of this time were active in investigating the origin of new individuals. And all their discoveries tended to weaken the belief in the theory of spontaneous generation.

Finally, the adherents of this theory were forced to restrict. their belief in spontaneous generation to the case of a few kinds. of animals, like parasites and the animalcules of stagnant water. It was maintained that parasites arose spontaneously from the matter of the living animal in which they lay. Many parasites have so complicated and extraordinary a life history that it was only after long and careful study that the truth regarding their origin was discovered. But in the case of every parasite whose life history is known, the young are offspring of parents, of other individuals of their kind. No case of spontaneous generation among parasites is known.

The same is true of the animalcules of stagnant water. If some water in which there are apparently no living organisms, however minute, be allowed to stand for a few days, it will come to be swarming with microscopic plants and animals. Any organic liquid, as a broth or a vegetable infusion exposed for a short time, becomes foul through the presence of innumerable bacteria, infusoria, and other one-celled animals and plants, or rather through the changes produced by their life processes. But it has been certainly proved that these organisms are not spontaneously produced by the water or organic liquid. A few of them enter the water from the air, in which there are always greater or less numbers of spores of microscopic organisms. These spores (embryo organisms in the resting stage) germinate quickly when they fall into water or some organic liquid, and the rapid succession of generations soon gives rise to the hosts of bacteria and Protozoa which infest all standing water. If all the active organisms and inactive spores in a glass of water are killed by boiling the water, "sterilizing" it, as it is called, and this sterilized water or organic liquid be put into a sterilized glass, and this glass be so well closed that germs or spores cannot pass from the air without into the sterilized liquid, no living animals will ever appear in it. It is now known that flesh will not decay or liquids ferment except through the presence of living animals or plants. To sum up, we may say that we know of no instance of the spontaneous generation of organisms, and that all the animals whose life history we know are produced from other animals of the same kind. "Omne vivum ex vivo," "All life from life."

The method of simple fission or splitting-binary fission it is often called, because the division is always in two-by which the body of the parent becomes divided into two equal parts -into halves-is the simplest method of multiplication. This is the usual method of Amaba (Fig. 120) and of many other of the simplest animals. In this kind of reproduction it is hardly exact to speak of parent and children. The children, the new Amabæ, are simply the parent cut into halves. The parent persists; it does not produce offspring and die. Its whole body continues to live. The new Amaba take in and assimilate food and add new matter to the original matter of the parent body; then each of them divides in two. The grandparent's body is now divided into four parts, one fourth of it forming one half