are very sensitive, and the polyps bearing them have for special function that of feeling or being sensible of stimuli from without. They are the sense organs or sense individuals of the colony. Finally, there are two other kinds of structures or individuals which produce the special reproductive cells for the perpetuation of the species. These are the modified medusa individuals, and one kind, larger than the other, produces the active sperm cells, while the other produces the inactive egg cells.

27. Increase in the degree of complexity.-In the corals, sea-anemones, and jelly-fishes there is plainly much more of a division of labor among the various parts of an individual and much more modification of these parts-that is, much more structural complexity than among the sponges and Hydra. And these, in their turn, are more complex than are the colonial Protozoa, the Volvocinæ. There is a great difference in degree of complexity among the slightly complex animals. But the various groups of these animals which we have studied can all be arranged roughly in a series beginning with the least complex among them and ascending to the most complex. And in this series, and in the always accompanying division of labor among the different parts, the gradual increase in complexity is beautifully shown.

From an animal composed of many structurally similar cells, each cell capable of performing all the life processes, we pass to an animal composed of cells of a few different kinds, of slight structural diversity. Each kind of cell devotes itself especially to a certain few life processes or functions. Next we find an animal in which the cells of one kind are specially aggregated to form a single part of the body which is specially devoted to the performance of a single function. This diversity among the cells increases, this aggregation of similar cells to form special parts or organs increases, and the division of labor or assignment of special functions to special organs becomes

more and more pronounced. Among the more complex polyps and jelly-fishes the contractile cells form distinct muscle fibers and muscles; the sensitive cells form distinct nerve cells and nerve fibers which are arranged in a primitive nervous system; the digestive cavity becomes. complex and composed of different portions; the reproductive cells are formed by special organs, and the distinction between the egg cells and the sperm cells—that is, between the female reproductive elements and the male reproductive elements-becomes more pronounced.

We have followed this increase or development of structural and physiological complexity from simplest animals to fairly complex ones. The principle of this development of complexity is evident. It will not be profitable to attempt to follow in detail this development among the higher animals. The complex animals are complex because their life processes are performed by special parts of their body, which parts are specially modified so as to perform these processes well. The animals which are more complex than those we have studied differ from these simply in the degree of complexity attained. In order to understand this better we shall not further consider special groups of animals, but special processes or functions, and attempt to see how the modification and increase in complexity of structure goes hand in hand with the increase of elaborateness or complexity in the performance of function.

CHAPTER III

THE MULTIPLICATION OF ANIMALS AND SEX

28. All life from life.-On the performance of the function of reproduction or multiplication depends the existence or perpetuation of the species. Although an animal may take food and perform all the functions necessary to its own life, it does not fulfill the demands of successful existence unless it reproduces itself. Some individuals of every species must produce offspring or the species becomes extinct. We have seen in our study of the simple animals that the function of reproduction is the first function to become differentiated in the ascent from simplest animals to complex animals. The first division of labor among the cells composing the bodies of the slightly complex animals and the first structural differences among the cells are connected with the performance of the function of reproduction or multiplication.

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 off

spring of parents of their kind. And in these latter days of microscopes and mechanical aids to observation it is even 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.

29. Spontaneous generation. 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 birth of Christ-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 laid 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