MIGRATION AND HEEMAPHRODITES. 37 1 



selves in a non-sexual manner by division, budding, for- 

 mation of spores, etc. All the great mass of Protista, the 

 Monera, Amoebae, 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. (Com- 

 pare chapter xvi.) 



It is self-evident, therefore, that Wagner's theory is quite 

 inapplicable to all those 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 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 



