Maech 29, 1907] 



SCIENCE 



501 



upon the problems of evolution, especially of 

 the origin and preservation of incipient 

 species, should take into consideration all 

 types of environment and all groups of organ- 

 isms. The discussion which has continued 

 for some time in the pages of this journal 

 with especial reference to the significance of 

 isolation on the origin of species has drawn 

 its data very largely from the distribution of 

 land and fresh-water vertebrates and has been 

 in the main carried on by systematists who 

 are specialists in these fields. But no insigni- 

 ficant part of the total organic evolution and 

 specific differentiation has been accomplished 

 in other phyla and in other environments. 

 The data of distribution from the littoral, 

 pelagic and abyssal areas of the sea, of the 

 microfauna and flora of fresh water, and of 

 the lower as well as the higher groups of 

 organisms, from widely ranging as well as 

 narrowly limited species and especially from 

 groups in which a high degree of specific 

 differentiation has been attained, such, for 

 example, as the Diatomaceae, the Eadiolaria 

 and the Coleoptera, all have some bearing on 

 the problem unless, as some of the physio- 

 logical school would have us believe, all wis- 

 dom is derivable only from pedigree cultures. 



Some familiarity with the microfauna and 

 flora of fresh water and with the pelagic 

 fauna of the Pacific has brought to my atten- 

 tion a series of facts which, with consider- 

 able uniformity, indicate a degree of coinci- 

 dent distrihution of related species which 

 stands in strong contrast with the wide-spread 

 isolation of related species and subspecies of 

 birds and mammals to which attention has 

 been drawn in previous discussions. 



The range of material available for demon- 

 stration of this tendency is indeed great. The 

 stimulus of the Kiel school of planktologists 

 has led to the investigation of scores, if not 

 hundreds, of lakes and streams, and the pub- 

 lication of extensive lists of their fauna and 

 flora. Recent oceanic explorations in the sev- 

 eral oceans have thrown a flood of light on 

 the distribution of pelagic organisms. 



The investigation of the microfauna and 

 flora of fresh water, both limnetic and littoral, 

 has demonstrated that its constituent organ- 



isms are to a very large degree cosmopolitan. 

 As an illustration of this fact one has but to 

 compare the lists from various localities. The 

 abundant species are very widely represented. 

 The number of rare species reported froni any 

 locality is largely a function of the thorough- 

 ness and the extent of the examination. 

 About 50 per ceot. of the protozoa and roti- 

 fers reported from the IlUnois Eiver have also 

 been reported from the Elbe, and about 60 

 per cent, of the rotifers from the Oudy. The 

 differential species, barring a considerable per- 

 centage resulting from synonymy, are almost 

 wholly of known wide range but of relative 

 rarity, and may be expected in most aquatic 

 environments of the north temperate zone on 

 long search. As a result of this cosmopolitan 

 distribution most of the known species of a 

 genus are wont to be found in a single 

 locality. An excellent illustration of this 

 phase of the question appears in Penard's 

 splendid monograph of the fresh-water 

 Rhizopoda in which he lists, from Geneva 

 alone, 216 of the 232 known species of this 

 group, and 91 of these from a single locality I 

 He found certain recognizable habitats, each 

 with its more or less peculiar fauna, such, for 

 example, as the sphagnum, the sylvicolous, and 

 abyssal faunas. These include, however, only 

 forty-nine species limited to any special 

 habitat. The genus Difflugia, for example, 

 with thirty-seven species, has but nine whose 

 distribution is restricted to a particular 

 habitat; the others may occur in any type of 

 habitat. 



In the autumnal months from September 

 to November, streams and lakes rich in 

 organic matter exhibit a marked increase in 

 the ciliate protozoa, especially of the Holo- 

 tricha. This is especially marked in the Illi- 

 nois River, and at such times many species 

 of the genera Prorodon, Lionotxis, Lacrymwria, 

 Didinium, Goleps, Nassula and Chilodon 

 occur coincidently. Species of other genera 

 of ciliates such as Stentor, Euplotes, Vorti- 

 cella, Zoothammium and Epistylis occur coin- 

 cidently at this season. Thus Dr. Volk has 

 recently found in the Elbe from September 9 

 to October 11, Y3 per cent, of the total number 

 of valid species, of the genera above named. 



