MIGRATION AND HERMAPHRODITES. 371 
selves in a non-sexual manner by division, budding, for- 
mation of spores, etc. All the great mass of Protista, the 
Monera, Amcebze, Myxomycetes, Rhizopoda, ete., 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. (Com- 
pare 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 tapeworms, 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 haying 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 
