310 DE. Gr. H. FOWLEE — BISCATAN PLANKTON : 



The Occurrence of closely allied Species in the same Water. 



It has been contended that, if the Darwinian canon of the Struggle for Existence be 

 well founded, closely allied species cannot co-exist in the same area ; the one is bound 

 to succumb to the other. The point is worth consideration in such a collection as the 

 present ; but the difficulty is to say what constitutes " close " alliance. In one sense, 

 all species of tlie same genus are closely allied to one another, even though they may be 

 as different morphologically as curta and daphnoides, and though their mere contour is 

 enough to show that they have responded differently to the stimulus of the Struggle. 

 Such forms are presumably in no more direct competition witli each other than with 

 other members of the plankton. 



Or, again, we may take as a standard those more subtle differences between the 

 species which Glaus expressed as subgenera and Miiller as " groups " ; these no doubt 

 bring together species of the closest morphological similarity, but take no account of 

 size. Thus, for example, by Miiller's grouping, spinirostris * with a male mean length 

 of 1"3 mm. is in the same group as magna with a male mean length of 2"91 mm. It is 

 not obvious that these two would come into direct competition, but the smaller species 

 will presumably compete with the younger stages of the larger. Such groupings by 

 morphological similarity seem, however, to be the best that we can take as a standard of 

 " close " alliance. 



The species captured by the ' llesearch ' are tabulated according to their respective 

 o-roups on pp. 228-9 ; and in discussing this question I must take it as granted that 

 my deductions t about their vertical distribution — summarised in the lower half of 

 PI. 27 — are approximately correct. 



As to the first pair of species — spinifera and inermis — nothing can be said, since the 

 latter was only observed in a haul which gave no clue to its distribution. The next pair, 

 however, —procer a and brachijaskos—ave interesting, since morphologically and in size 

 they are very close to one another ; procera seems to reach down to about 250 f m. with 

 a plurimum at 100 fm., brachyaskos to reach up to about 300 fm. with a plurimum about 

 450 fm. ; they apparently did not coexist in the same waters. Then come roiundata and 

 pusilla, morphologically more easily distinguishable than the last pair, but of like size : 

 the younger stages of roiundata were probably confined to the upper 200 fathoms, but 

 the oldest stage ap^Darently overlapped pusilla to some extent ; on the other hand, the 

 plurimum of the former was between 600-4<00 fm., that of the latter between 750- 



500 fm. 



Next is the magna-^xow^, represented by at least three species, spinh^ostris being in 

 many points distinct and probably out of place here. Of these three, magna and zetesios 

 are of close morphological similarity and both of large size ; they certainly seemed to 

 inhabit the same water, and to belong to the epiplankton and upper mesoplankton. On 

 the other hand, hyalophyllum, though rising into the epiplankton at night, seemed to 



* Miiller only places this species provisionally in the Jiiar^nn-group ; it seems to me to have more (though 

 uncertain) afTmity with the loricata-gxouj). 



■\ They were made and PI. 27 drawn before this paragraph was thought of. 



