■504 



SCIENCE 



[N.S. Vol. XXV. No. 639 



-workers on this genus. They are the cosmo- 

 -politan C. furca (Ehrbg.), and C. lineatum 

 ■■(Ehrbg.), and the minute 0. eugrammum 

 (Ehrbg.). The species are evidently closely 

 related, differing only in certain proportions 

 of the midbody, in the degree of ventral ex- 

 'jeavation and in size. The examination of a 

 ^wide range of material and a statistical study 

 •of their proportions has convinced me of their 

 ■distinctness. Their resemblances are so 

 plainly marked that one is forced to conclude 

 that they are branches of a common stem or 

 -«o-mutants. G. furca and C. lineatum are 

 -widely distributed in temperate and tropical 

 ■«eas and the occurrence of 0. eugrammum in 

 4oth the Mediterranean and in the Pacific is 

 suggestive of a similar range. Its small size 

 ■enables it to slip through the mesh of even 

 A fine plankton net. This fact, together with 

 its resemblance to C. lineatum, doubtless ex- 

 -plains its comparative absence in published 

 Tecords of distribution. These species are 

 ■abundant at the same seasons, and are taken 

 Tepeatedly at the same levels in the same net. 

 Their pelagic habit is suggestive of a pelagic 

 •origin and their coincident distribution over 

 -wide areas would seem to afford abundant and 

 -oft-repeated opportunities for swamping out 

 incipient species by interbreeding of the 

 ■diverging forms with parental stock, provided 

 their origin took place by the gradual accumu- 

 lation of minute fluctuating variations. 



The lists of plankton organisms in Euro- 

 --pean seas published by the Conseil permanent 

 pour I'exploration de la mer, as well as our 

 records for the past four years at San Diego, 

 show many instances of the coincident dis- 

 tribution of Ceratium macroceros, 0. longipes, 

 -and 0. intermedium, three closely related 

 species, all having open tips to the antapical 

 horns and more or less postindentation, but 

 -differing from each other in size, form of the 

 shoulders and the angular divergence of the 

 horns. These species are so closely related 

 that they have been much confused in the 

 past, but our best specialists in this group are 

 -now agreed that the species are distinct. 

 They are the most closely related species of 

 their section of the genus to which they be- 

 long and they have a coincident and appar- 



ently coextensive distribution in temperate 

 seas. 



Another striking instance of coincident dis- 

 tribution is found in the species of the 0. 

 tripos group, including the closely related G. 

 azoricum, 0. arcuata, G. heterocampium, 0, 

 hucephalum and G. curvicorne, which are very 

 often found together in the plankton of the 

 Pacific and extensively throughout much of 

 the Atlantic as Cleve has shown in his ' Sea- 

 sonal Distribution of Atlantic Plankton Or- 

 ganisms.' 



Dividing twenty-seven of the more common 

 species of Geratium in groups of most closely 

 related species and determining their distribu- 

 tion in surface waters of the Pacific in twenty- 

 one typical collections, we find that the per- 

 centage of coincident distribution of the 

 species of the several groups actually realized 

 in the collections ranges from 48 per cent, to 

 68 per cent, on an average. 



Unfortunately for my discussion, we know 

 neither the strata nor the seasons in which 

 the sexual reproduction of these organisms 

 occurs, if, indeed, any exists. 



It will doubtless be suggested by defenders 

 of the universal potency of isolation in the 

 origin of species that the instances of coinci- 

 dent distribution of related species which I 

 have here cited are those of organisms which 

 multiply predominantly by asexual methods or 

 at least by parthenogenesis, that sexual repro- 

 duction is entirely, or at least relatively, absent 

 and that this in itself constitutes an effective 

 isolation and prevents the swamping out of 

 new forms. In reply to this attention should 

 be directed to the admirable researches of Dr. 

 Schaudinn and of Professor Richard Hertwig, 

 and especially to the results of investigations 

 into the life histories of the Sporozoa and the 

 Foraminifera which have brought to light the 

 wide-spread occurrence of typical sexual repro- 

 duction in so many of the protozoa. So stri- 

 king have been these results that it is safe to 

 say that a full knowledge of life histories wiU 

 reveal sexual reproduction in all protozoa. 

 Over against the apparent infrequency of 

 sexual reproduction in the protozoa and many 

 fresh-water organisms should be placed the 

 brevity of the life cycle and the considerable 



