Xf: SPAWNING HABIT OF TELEOSTEAN FISHES. 383 



on, they, like the young plaice, are found close inshore, in the littoral 

 region. 



It is seen that the life-histories of these two common fishes agree 

 in this particular, that the eggs and larvae, before arriving at the 

 adult stage, have to pass through many vicissitudes and changes of 

 environment, which in their general features are not accidental, but 

 are determined in their order of succession for each species. These 

 changes we may term the ' ontogenetic migration.' ' The first and 

 most marked difference* of environment is that of the egg itself, and 

 the two general types are illustrated by the plaice and the herring, 

 i.e., the pelagic and demersal. The pelagic egg floats free (rarely in 

 masses) in the open water ; the demersal is usually fixed to foreign 

 bodies on the sea-bottom. All the gadoids and pleuronectids appear 

 to have pelagic eggs, whilst the clupeoids vary greatly in type, and 

 the smaller acanthopterous littoral fishes have demersal eggs. 



It would be well to inquire into the meaning of these two types 

 of eggs and the remarkable difference of environmental conditions 

 involved ; and a careful review of the facts will convince one that the 

 pelagic spawning-habit is the more primitive, and is probably to be 

 interpreted upon phyletic grounds, whereas the demersal has been 

 secondarily acquired in the course of time. A few of the considera- 

 tions leading to this conclusion may be here given : — 



1. The pelagic-spawning fishes are, almost without exception, 

 more prolific : their fecundity may be expressed by as high a figure 

 as 150,000,000 (in the ling), a number beyond all human compre- 

 hension. 



2. They have more primitive methods of fertilisation and no 

 sexual dimorphism : the eggs and sperms are simply extruded into 

 the water, and there is no ' pairing.' 



3. Their methods of reproduction are primitive : there is an 

 extended period of ripening, and after the extrusion of the sexual 

 elements, the demand of the future generation upon the parent comes 

 to an end ; whereas in the demersal-spawning types we have, amongst 

 numerous instances, the nest-building habits of the stickleback and 

 its parental devotion to the young, the jealous watching of the lump- 

 sucker, and the careful selection of sites for oviposition by the gobies, 

 features which culminate in the viviparous habit of the blennies. 



4. The ontogeny in the demersal types is more protected, and 

 the larvae are usually hatched at a much later stage than is the case 

 in the pelagic types. 



5. There is no conceivable advantage to be obtained by the 

 secondary adoption of a pelagic spawning-habit, but there is in the 

 secondary assumption of a demersal spawning-habit. 



These considerations are all correlated, but they form strong 



^For method of determining exactly this migration, see A.T.M., Rep. Fish. 

 Board Scotland, xiv., p. 294, 1896 ; and Ann. Mag. Nat. Hist., ser. 6, vol. xvi., pp. 285- 

 288, October. 1895. 



