500 SCIENCE PROGRESS. 



very early stage in which the larva has to begin to obtain its 

 food. The early use of the jaws, muscles, and sense-organs 

 would cause their precocious development, so that the 

 quantity of mesoblast surrounding- the yolk-sac would be 

 reduced, and thus the veins could no longer at this stage be 

 divided from each other. 



No Ichthyologist doubts that the fishes with closed air- 

 bladders have been derived from forms in which the bladder 

 opened from the gullet or other part ot the intestines. The 

 Physostomi retain other primitive characters besides the 

 opening of the air-bladder, for example, the posterior 

 position of the anus and of the pelvic fins. In the Anacan- 

 thini and Acanthopterygii the position of the anus is more 

 anterior, and the pelvic fins instead of being placed in 

 the hinder region of the body are, in most cases, either 

 below or actually in front of the pectoral fins. According 

 to the law of recapitulation we should expect to find that the 

 larvie of the two groups were similar in the position of the 

 anus and the origin of the pelvic fins. But this is not the 

 case. In the physostomous larva the anus is near the end of 

 the tail, and separated by a considerable interval from the 

 yolk-sac, while in the physoclist larva the rectum and anus 

 are immediately behind the yolk-sac, and far in front of the 

 end of the tail. The explanation here appears to be that 

 in the adult the anus has been pushed forward by the 

 increased use and consequent increased development of the 

 ventral fin (so-called anal fin), and that the new condition 

 is developed directly because there is no influence in larval 

 life tending to retain the ancestral condition. The ontogeny 

 of the j3elvic fins in the Physoclisti is more recapitulative, 

 but not completely so. The fins when they first appear are 

 small dermal folds behind the pectoral fins, but they very 

 soon pass to the position which they occupy in the adult. 

 It is not very easy to discern the probable causes of the 

 alteration in the position of the pelvic fins either on 

 Darwinian or Lamarckian principles, but we may reasonably 

 suppose that the particular position of these fins in the 

 adult has been determined by certain requirements of atti- 

 tude and mo\ ement in the fish. The mechanical conditions 



