382 Causes and Course of Organic Evolution 



The Proteomyxa, while including some doubtful types, 

 contain several species that find their nearest allies of a more 

 advanced type, amongst the fresh-water foims of the three 

 succeeding groups. The Lobosa and Mycetozoa seem equally 

 to have been derived along with members of the Proteomyxa, 

 from primitive myxobacterial forms, that gradually evolved 

 a nucleated and sexual condition. Most of these it is true 

 can resist drying, and so might be carried in dust-winds or 

 as spores from seashore to inland situations, or be left stranded 

 from the sea in lagoons. But we should expect to find as 

 rich or even richer a variety still living in the sea or along its 

 shores, had the inland forms been of sea-derivation. The 

 plastogamy or multiple fusion of young cell-individuals, so 

 strikingly seen in the Mycetozoa, is observed outside the group 

 almost wholly in fresh-water types like Difflugia and Cen- 

 tro pyxis. 



The simpler forms of Heliozoa show decided affinities ^ath 

 the fresh-water Proteomyxa and Lobosa, while such specialized 

 fresh- water types as Acanthocystis and Pinaciophora approach 

 very nearly to the marine Radiolaria, and include a few species 

 that may be marine. We believe that many will concede 

 a probable fresh-water ancestry alike for the marine groups 

 Foraminifera and Radiolaria. For the genera Difflugia, Arcella, 

 and Phryganella are all fresh-water forms, or brackish-water 

 transition forms, from the Lobosa to foraminiferal types like 

 Pseudodifflugia, Gromia, and Lagena, that are fresh-water, 

 brackish, or marine. In like manner Trinema, Acanthocystis, 

 and Euglypha, though shomng little if any indication of an 

 internal capsule, approach very near to the simplest Radio- 

 laria, like the group Physematiidse. On the other hand no 

 known simple marine types lead up by graded steps to either 

 of the large polymorphic marine groups, Foraminifera and 

 Radiolaria. 



But, if we assume that ancestors of both were derived from 

 a fresh-water source, the open free swimming life and chem- 

 ically active sea water that they eventually reached would 

 act as a double stimulus in addition to those of light, gravity, 

 and other energies, so as to evolve them into that wealth of 

 species, variety of pelagic contrivance, and complexity of 

 structure that are now so characteristic of most of them. But 

 we would by no means consider, as some have done, that such 

 changes took place mainly at great ocean depths. The sea 

 surface or the comparatively shallow littoral zone are the marine 

 regions where actions and reactions would always be most ex- 

 pressed, and so where species would evolve most plentifully. 



