312 Causes and Course of Organic Evolution 



the Ulvacese from simpler tetrasporaceous ancestry, the writer 

 considers that the first striking evidence is got amongst the 

 green algie of migration from fresh through brackish into 

 salt water. For, while the Tetrasporacese include 11 genera 

 and 'iS species that are fresh-water, and 1 genus with 2 species 

 that are marine, the Ulvacese include 5 genera and '^9 species 

 that are fresh-water, also 6 genera and 49 species that are 

 brackish or marine. It might however be argued that all 

 have been marine primitively, and have gradually sent motile 

 gonidia into brackish estuaries, which there established their 

 respective species, while later they again migrated into fresh 

 water. So many and so grave objections can be offered at 

 once to such a suggestion that we do not propose at this stage 

 to linger over it further. 



The Ulvacese then may well be designated a transition fam- 

 ily, but one which traces fresh-water descent. It is one also 

 which presents many transition conditions in some of its genera 

 and species. Thus Monostroma hullosum has been regarded 

 by Kutzing rather as a Tetraspora and is wholly fresh-water, 

 while related species, vdth. more ample thallus, are brackish 

 or marine. M. Grevillei — "VN-idely known also as Ulva Lactuca — 

 is marine or subbrackish and of vdde geographic range over 

 the world. Ulva, in restricted sense, includes brackish and 

 salt-water species, but Enteromorpha — which like Ulva is 

 world-^^'ide — includes not a few fresh- water species. The genus 

 Ilea is of interest, in that it closely resembles Enteromorpha, 

 but has developed a brownish color. The two species that 

 make up the genus Letter stedtia are the most highly modified 

 members of the family, and are now purely marine. From 

 their occurrence at Port Natal and in Australia thej^ may have 

 become fixed in their environal relations during cretaceous times, 

 and spread to both areas when shallow seas united both regions. 



The Ulothricacese, Hydrodictyacese, Mycoidacese, Oedogo- 

 niacese, Cladophoracese, and Vaucheriacese are all important 

 green algoid families whose purely fresh-water, or preponder- 

 atingly fresh-water, origin and distribution seem unquestion- 

 able. A glance at the table (p. 305) makes this at once evident. 

 But we can well afford to linger somewhat over the group 

 Cladophoracese. It includes six genera and about 200 species, 

 of which one genus Spongocladia with 3 species is marine, 

 another Pithophora with 8 species is fresh- water, while the 

 remaining ones like Cladophora and Rhizoclonium, that are 

 world-Ande in distribution, contain species wjiich occur in 

 fresh, in brackish, or in salt water. So this again may truly be 

 designated a transition family. 



