Now, from the earliest times of the organic history of the earth, there have existed thousands of organic species (thousands of which still exist) in which no difference of sex whatever exists, and, in fact, in which no sexual propagation takes place, and which exclusively reproduce themselves in a non-sexual manner by division, budding, formation of spores, etc. All the great mass of Protista, the Monera, Amœbæ, Myxomycetes, Rhizopoda, etc., in short, all the lower organisms which we shall have to enumerate in the domain of Protista, standing midway between the animal and vegetable kingdoms, propagate themselves _exclusively in a non-sexual manner_. And this domain comprises a class of organisms which is one of the richest in forms, nay, even in a certain respect the richest of all in forms, as all possible geometrical fundamental forms are represented in it. I allude to the wonderful class of the Rhizopoda, or Ray-streamers, to which the lime-shelled Acyttaria and the flint-shelled Radiolaria belong. (Compare chapter xvi.) It is self-evident, therefore, that Wagner’s theory is quite inapplicable to all these non-sexual organisms. Moreover, the same applies to all those hermaphrodites in which every individual possesses both male and female organs and is capable of self-fructification. This is the case, for instance, in the Flat-worms, flukes, and tape-worms, further in the important Sack-worms (Tunicates), the invertebrate relatives of the vertebrate animals, and in very many other organisms of different groups. Many of these species have arisen by natural selection, without a “crossing” of the originating species with its primary form having been possible. As I have already shown in the eighth chapter, the origin of the two sexes, and consequently sexual propagation in general, must be considered as a process which began only in later periods of the organic history of the earth, being the result of differentiation or _division of labour_. The most ancient terrestrial organisms can have propagated themselves only in the simplest non-sexual manner. Even now all Protista, as well as all the countless forms of cells, which constitute the body of higher organisms, multiply themselves only by non-sexual generation. And yet there arise here “new species” by differentiation in consequence of natural selection.