380 Causes and Course of Organic Evolution 



representatives of primitive Radiolaria. For the stiff pseudo- 

 podia and central capsule are more or less foreshadowed in 

 these apparently transition groups. All of the above are 

 fresh-water, or include species that pass from fresh-water to 

 marine life. 



The Sporozoa, that might well be viewed as a group inter- 

 mediate between the Lobosa and the next group the Masti- 

 gophora, is at the present day exceptionally rich in genera and 

 species. This is in part probably due to their being often 

 free at one time and parasitic at another, and both under a 

 great variety of circumstances. It is very difficult at the 

 present stage of our knowledge of these organisms to reach 

 an exact statistical position, but from comparison of the mono- 

 graphs of leading writers on these we would consider that 

 there are about 60 genera of land parasites, 5 neutral, and 40 

 genera of marine parasites. 



The Hsemoflagellata includes 8 or 9 genera, of which 6 are 

 wholly or chiefly land or fresh-water parasites, two of the 

 genera are either marine parasites or include a fair number 

 of marine forms. 



The Mastigophora or flagellate Infusoria is a group that 

 is exceptionally rich in genera and species, though some of 

 these may prove to be doubtful or valueless when the com- 

 plete life cycles are known. Since the fresh-water forms have 

 received so much attention by microscopists, and the marine 

 ones have been less minutely examined, it is likely that the 

 number of genera and species in the latter will be considerably 

 increased. As delimited at present however there are 94 

 fresh-water genera, 13 that are common to fresh and salt 

 water, and 36 that are purely marine. Butschli (93) consid- 

 ered that the group included from 450 to 500 species. In our 

 calculation of the genera, the Chlamydomonidse have been 

 excluded as having been already reckoned amongst the algae. 



The cihate Infusoria show, like the last, similar diversity 

 of structure, variety of genera, and richness in species. Ninety- 

 five genera are fresh-water or rarely are land parasites, 31 are 

 common to fresh and brackish water, while 24 are purely 

 marine. 



The above results are tabulated on the opposite page. 



In \'iew of the above statistics the question may well be 

 asked for the simpler animals, as we already have for plants, 

 whether these originally evolved over a lacustrine and more 

 or less landlocked, or over a marine, area. The latter view 



