68 



investigation, and that such assemblages do occur follows from 

 the consideration that fertilization is external and pelagic and 

 especially from the consistent results from year to year of the 

 distribution from them. It may be added that the difference 

 in habit of the males and females is a general one and is not con- 

 fined to fishes. In my paper on the generative organs of the 

 porpoise I gave reasons . for supposing that the sexes behaved 

 just as they have been proved to do in fishes.* 



There are one or two points of more theoretical interest which 

 may be briefly touched upon before we leave the consideration 

 of the blood control of the nervous system and internal secretions. 



Fishes are conspicuous^ contranatant in the adult condition, 

 and especially when migrating for spawning. In the young state 

 they are denatant, and the denatation is not confined to the drifting 

 period, but still dominates the movements even when the power 

 of contranatation has been acquired. Contranatation first 

 definitely emerges amongst the fishes and is not thereafter lost, 

 but amongst the groups below fishes it has only been discovered 

 by the larger and active Mollusca and by the Crustacea. The rest 

 are permanently denatant whether they be holoplanktonic or 

 meroplanktonic. The interesting feature which is to be remarked 

 is that in fishes and in Crustacea and probably also in Cephalopoda, 

 which have become capable of contranatation, the stimulus to a 

 contranatant migration is provided. 



Ontogenetically and phylogenetically then contranatation 

 arises from denatation. During the denatant period the larvre 

 tend to congregation also, and we are led to enquire whether 

 this is due to the accident that they have emerged from the same 

 area, and are carried by the same currents to the same place or 

 to an agency which promotes the congregation. The same general 

 fact is true of holoplankton and mero plankton. Whilst admitting 

 the trend of the common factors in bringing about the congrega- 

 tion, it is probable that the tendency to crowd together is clue to 

 the action of a primitive hormone which is not at all unlike that 

 of the spawning migration, a primitive physiological attraction, 

 as old as cell division and dating from the Protista. 



Whether this be the fact or not, it is obvious that at all stages 

 fish and their predecessors are responsive to environment. The 



1918, Jour, of Anatomy, v. 52, p. 197, 



