L'TG THEORIES ON THE ORIGIN OE VERTEBRATES 



be the most primitive living Vertebrates and yet show no 

 particular anneliclan affinities ? Dohrn tries to answer this 

 awkward question by showing that these forms are not 

 primitive but degenerate. He points out first that Cyclo- 

 stomes are degenerate fish, half specialised and half degraded 

 in adaptation to a parasitic mode of life. He thinks that if 

 an Ammocoetes were to become sexually mature and de- 

 generate still further, forms would result which would 

 resemble Amphioxus, and ultimatel)', if the process of de- 

 generation went far enough, larval Ascidians. Amphioxus 

 therefore might well be considered an extremely simplified 

 and degenerate Cyclostome, and the ascidian larva the last 

 term of this degeneration-series. Both Amphioxus and the 

 Ascidians would accordingly be descended from fish, instead 

 of fish being evolved from them. 



Dohrn conceived that the transformation of the Annelid 

 into the Vertebrate took place mainly by reason of an 

 important transforming principle, which he calls the principle 

 of function-change. Each organ, Dohrn thinks, has besides 

 its principal function a number of subsidiary functions which 

 only await an opportunity to become active. "The trans- 

 formation of an organ takes place by reason of the succes- 

 sion of the functions which one and the same organ possesses. 

 Each function is a resultant of several components, of which 

 one is the principal or primary function, while the others are 

 the subsidiary or secondary functions. The weakening of 

 the principal function and the strengthening of a subsidiary 

 function alters the total function ; the subsidiary function 

 gradually becomes the chief function, the total function 

 becomes quite different, and the consequence of the whole 

 process is the transformation of the organ" (p. 60). 

 Examples of function-change are not difficult to find. Thus 

 the stomach in most Vertebrates performs both a chemical 

 and a mechanical function, but in some forms a part of it 

 specialises in the mechanical side of the work and becomes a 

 gizzard, while the remaining part confines its energies to the 

 secretion of the gastric juice. So, too, it is through function- 

 change that certain of the ambulatory appendages of Arthro- 

 pods have become transformed into jaws their function as 

 graspers of food has gradually prevailed over their main 



