Mabch 29, 1907] 



SCIENCE 



503 



and contemporaneous sexual reproduction in 

 five specie or varieties, in this instance, with 

 the possibility that the remaining eight may 

 also be in sexual activity. There is little 

 possibility of any horizontal stratification 

 separating the species here, as the waters of 

 the Illinois River are shallow (three to ten 

 feet) and are stirred by current and diurnal 

 movements. These same species occur to- 

 gether, moreover, in shallow bottom-land lakes 

 but a few inches in depth. 



Closely related species and varieties of 

 Brachionus thus live together in the same 

 habitat and breed at the same times, with no 

 external barriers to prevent their swamping 

 out. 



The fresh-water Copepoda afford an excel- 

 lent test for the contemporaneous breeding of 

 closely related species having coincident dis- 

 tribution, since the eggs are carried for some 

 time by the female during their development 

 and the breeding periods of the several species 

 can thus be determined. There are six species 

 in the subgenus Cyclops which occur in the 

 Illinois River and are often found breeding in 

 the same seasons. Two of the most closely 

 related, 0. edax Forbes and 0. leucharti Glaus, 

 are found breeding coincidently in August. 

 The other members of the subgenus, G. viridis 

 Jurine and 0. hicuspidatus Glaus have over- 

 lapping breeding periods in the spring, and 

 the two varieties of G. viridis, v. insectus 

 Forbes and v. hrevtspinosus Herrick breed 

 coincidently throughout the summer. C. ser- 

 ridatus Fischer and G. prasinus, closely re- 

 lated species of the subgenus Eucyclops have 

 a wide distribution in the United States and 

 Europe and are frequently found in the same 

 collections. Diaptomus pallidus and D. sici- 

 loides, two most closely related species of 

 Oalanidae, are found together in about equal 

 numbers, breeding at the same seasons in the 

 Illinois River. 



The limnetic fauna and flora thus afford 

 numerous instances not only of the coincident 

 distribution of closely related species, but as 

 well of their contemporaneous breeding. The 

 extent to which this factor of contem- 

 poraneous breeding prevails among the or- 

 ganisms of fresh waters and the fuU force 



of its bearing upon the problem of isolation 

 are best understood when the fact is borne in 

 mind that the whole ensemble of organisms 

 constituting the plankton, runs throughout 

 the year, year in and year out, this rhythmical 

 series of recurrent cycles. In this most, if 

 not all, of the constituent plants and animals 

 have coincident periods of rapid increase fol- 

 lowed by one of subsequent declines offering, 

 in cases of sexual reproduction, repeated op- 

 portunities for the interbreeding of related 

 forms, and the swamping out of incipient 

 species arising by the slow accumulation of 

 minute fluctuating variations. There ap- 

 pears, however, to be no lack of specific differ- 

 entiation among these organisms. Isolation 

 by external factors seems to play a small part, 

 if indeed any, in specific differentiation in this 

 type of habitat. 



The pelagic life of the sea abounds in seem- 

 ing instances of the coincident distribution 

 of closely related species in an environment 

 presenting a minimum degree of diversifica- 

 tion and a relative absence of barriers. Tha 

 incompleteness of our knowledge of the hori- 

 zontal and vertical distribution of pelagic 

 organisms and of their breeding seasons leaves 

 here much to be desired in the data for a dis- 

 cussion of this problem. There is great spe- 

 cific differentiation among pelagic diatoms, 

 Radiolaria, Foraminifera, Dinoflagellata; 

 Gopepoda and Amphipoda. Frequently many 

 species of the same genus occur in the same 

 region, witness the fifty species of Goscino- 

 discus found by Karsten in the Antarctic 

 plankton of the Valdivia Expedition, largely 

 at a depth between 60 and 40 meters. 



The genus Geratium of the dinoflagellates 

 is one of the most diversified and its species 

 are among the most variable of marine organ- 

 isms and are mainly of wide distribution. In 

 the plankton of the Pacific immediately at the 

 surface, that is in the upper 2-3 meters, it is 

 no unusual thing to take 25-30 species in a 

 single 14-inch net. 



There are three species of Geratium which 

 are usually coincidently abundant in the 

 Pacific off San Diego which are most closely 

 related, so closely in fact that their distinct- 

 ness has frequently been overlooked by casual 



