386 



NATURAL SCIENCE. 



June, 



Let us examine the conditions of pelagic drift along A B by means 

 of two perpendicular sections (Figg. 2 and 3). In Fig 2. are seen the 

 conditions in a shallow-water area, and in Fig. 3, those in an area in 

 which the coast slopes off suddenly into deep-water. The point A is 

 supposed to represent the spawning-area, and the eggs are drifted 

 along A B till at B the young fishes migrate downwards to the bottom, 

 which is, in the ideal case, at a suitable depth for them (in this 

 particular instance, supposed to be from 5 to 10 fathoms). Then it is 



Fig. 2. — Pelagic Drift in a Shallow-water Area. 

 Fig. 3. — Pelagic Drift in a Deep-water Area. 



evident that the length of A B must, for the survival of the species, be 

 between the limits A B^ and A B^, as all the larvae, which reach the 

 bottom sooner or later than this, are in an environment to which they 

 are not adapted, and in which they would therefore, in the main, 

 succumb. In Fig. 2, where the conditions of a gradually-shelving 

 shore are shown, the limits B^ B^^ are wider apart and this may largely 

 explain why slowly-shelving shores and friths are selected by 

 spawning-fish as nurseries for their young in preference to areas with 

 deep water close in shore. The limits of B^ B^,, and hence of the 

 position of A, are therefore determined, on the one hand, by the 

 bathymetric conditions, and on the other, by the adaptability of the 

 young forms. Suppose the existence of a species in which the young 

 were adapted for living in all the environments which obtain in from 

 5 to 15 fathoms, then in the area represented by Fig. 2 the species 

 would be able to spawn successfully over a very extended area. 



Again, if a species were adapted to a general distribution at 

 various depths, and this adaptability were to be reflected back to its 

 young stages on their assuming a bottom-habitat, then the position of 



