Influence of Environment on Animals 207 



the two eyes of a fish embryo {Fundulm heteroditus) to fuse into 

 a single cyclopean eye through the addition of magnesium 

 chloride to the sea-water. When he added al)out G grams of 

 magnesium chloride to 100 c.c. of sea-water and i)laced the 

 fertilized eggs in the mixture, about 50 per cent of the eggs gave 

 rise to one-eyed embryos. 



When the embryos were studied the one-ej'-ed condition was found 

 to result from the union or fusion of the ''Anhigen" of the two eyes. 

 Cases were observed which showed various degrees in this fusion; it 

 appeared as though the optic vesicles were formed too far forward 

 and ventral, so that their antero-ventro-median surfaces fused. Tliis 

 produces one large optic cup, which in all cases gives more or less 

 evidence of its double nature.^ 



We have confined ourselves to a discussion of rather simple 

 effects of the change in the constitution of the sea-water upon 

 development. It is a priori obvious, however, that an unlimited 

 number of pathological variations might be produced by a 

 variation in the concentration and constitution of the sea-water, 

 and experience confirms this statement. As an example we 

 may mention the abnormalities observed by Herbst in the 

 development of sea-urchins through the addition of lithium to 

 sea-water. It is, however, as yet impossible to connect in a 

 rational way the effects produced in this and similar cases with 

 the cause which produced them; and it is also iuipossible to 

 define in a simple way the character of the change produced. 



III. THE influence OF TEMPERATURE 



a) The influence of temperature upon the density of pelagic 

 organisms and the duration of life. — It has often been noticed 

 by explorers who have had a chance to comj^are the faunas in 

 different climates that in the polar seas such species as thrive 

 at all in those regions occur, as a rule, in much greater density 

 than they do in the moderate or warmer regions of the ocean. 

 This refers to those members of the fauna which live at or near 



1 Stockard, Archiv f. Entwicklungsmechanik, XXIII, 249, 11)07. 



