250 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



of 13.3° in 25 fathoms. This is without exception the most rapid 

 change in temperature recorded at any of the twenty-seven stations at 

 which serial temperatures were taken on the expedition. It occurs, 

 moreover, within the strata of water which normally is abundantly in- 

 habited by Ceratium, and in amplitude it corresponds to changes in 

 surface temperatures to be found only within 30 or 40 degrees of 

 latitude. 



The contrast in specific gravity of surface waters at this station 

 (1.0256) and that (1.0252) of the adjacent one (4712) farther in the 

 Humboldt Current is also extreme, though similar to the change found 

 elsewhere along the western and southwestern edge of the current. 



No less striking than the rapidity in change in temperature and den- 

 sity is the character of the plankton in this region (see Agassiz, 1906, 

 p. 18). There was an unusual proportion of dead and moribund mate- 

 rial, and the debris of plankton organisms, skeletons of dinoflagellates, 

 diatoms, radiolarians, and fragments of copepods. The explanation of 

 this rise of the isothermobath of 60° on the western margin of the cur- 

 rent (see Agassiz, 1906, plate 8) and the accompanying moribund na- 

 ture of much of the plankton is possibly to be sought in some phase of 

 vertical circulation within the strata affected by the currents, such as an 

 aspiration zone or an upward compensation movement at the edge of the 

 current due to the piling up of water on its left margin as a result of the 

 earth's rotation (see Nathanson, 1906, 1906 a), or to an upward suction 

 between local divei'ging branches of the current (see Schott, 1903). 



The location of this station with reference to the current and to the 

 distribution of temperatures in the vertical direction is thus unique in 

 the extremes of the environmental conditions afforded within relatively 

 narrow limits in the zone inhabited by the phytoplankton. 



That the shock of environmental changes upon the organism at the 

 time of reproduction is potent in producing changes in form or muta- 

 tions has been shown experimentally by Tower (1906) for Leptinotarsa. 

 The changes noted by Calkins (1906) for Paramecium followed the isola- 

 tion of a conjugating pair of individuals which doubtless involved some 

 environmental change. The experiments of MacDougal, Vail, and Shull 

 (1907) upon Oenothera indicate the potency of external agencies in pro- 

 ducing mutations in plants. 



The occurrence of these heteromorphic chains in regions where they 

 may have been subjected to unusual environmental conditions is, I be- 

 lieve, one consideration for regarding them as mutants called forth by 

 the shock of environmental contrasts. The autotomy of the horns in 



