On these grounds we should agree with Haeckel in distinguishing the living world into the Metazoa, or Higher Animals, which {41}are sharply marked off; the Metaphyta, or Higher Plants, which it is not so easy to characterise, but which unite at least two or more vegetal characters; and the Protista, or organisms, whose differentiation is limited to that within the cell (or apocyte), and does not involve the cells as units of tissues. These Protista, again, it is impossible to separate into animal and vegetal so sharply as to treat adequately of either group without including some of the other: thus it is that every text-book on Zoology, like the present work, treats of certain Protophyta. The most unmistakably animal group of the Protista, the Ciliata, is, as we have seen, by the complex differentiation of its protoplasm, widely removed from the Metazoa with their relatively simple cells but differentiated cell-groups and tissues. The line of probable origin of the Metazoa is to be sought, for Sponges at least, among the Choanoflagellates (pp. 121 f. 181 f.). {42}CHAPTER II PROTOZOA (_CONTINUED_): SPONTANEOUS GENERATION—CHARACTERS OF PROTOZOA—CLASSIFICATION THE QUESTION OF SPONTANEOUS GENERATION From the first discovery of the Protozoa, their life-history has been the subject of the highest interest: yet it is only within our own times that we can say that the questions of their origin and development have been thoroughly worked out. If animal or vegetable matter of any kind be macerated in water, filtered, or even distilled, various forms of Protista make their appearance; and frequently, as putrefaction advances, form after form makes its appearance, becomes abundant, and then disappears to be replaced by other species. The questions suggested by such phenomena are these: (1) Do the Protista arise spontaneously, that is, by the direct organisation into living beings of the chemical substances present, as a crystal is organised from a solution: (2) Are the forms of the Protista constant from one generation to another, as are ordinary birds, beasts, and fishes?