SALPA IN RELATION TO EVOLUTION OF LIFE. 179 



With each increase in size, the habit of visiting the bottom must 

 itself have become more aixi more fixed, until the life upon the 

 bottom, which may have been at first only intermittent and more 

 or less accidental, at last became established in the ancestors of the 

 ascidians as a constant characteristic peculiarity. 



As this new mode of life was gradually acquired, some method 

 of aerating the fluids of the body must also have been gradually 

 evolved ; for without it, a minute animal adapted for a free active 

 life in the highly aerated surface-water, could not, at the same time 

 that it grew larger, acquire a less active habit of life in the bottom 

 strata where the water is less perfectly aerated, the products of de- 

 composition of organisms more concentrated, and the capacity for 

 passing from exhausted and impure water to a fresh environment, 

 restricted both by the more stationary habit and by the fact that 

 life in space has been exchanged for a home which is limited by a 

 surface. 



Undoubtedly the change of habit was accompanied by the gradual 

 perfection of the system of blood-spaces around the pharynx, which, 

 at first indefinite and irregular, became constant on the margins of 

 the pharyngeal clefts, which thus gradually acquired a new function 

 and became gill-slits, and also became duplicated as the animals 

 grew larger and the need for more perfect respiration increased 

 with their chan2;e of habits. 



I hope that no one will interpret the last sentence as an expression 

 of the belief that the need for respiration caused the gill-slits to 

 multiply. I believe, and shall try to show further on, that the 

 tendency to duplicate a structure, either radially, bilaterally or 

 serially, is a result of the method of growth by cell multiplication, 

 and that in the case in question the serial reduplication has been 

 fixed and preserved by natural selection on account of its value in 

 respiration. 



The context shows that I also regard the gill -slits of vertebrates 

 and those of tunicates as homologous structures inherited from a 

 common source, the primary pharyngeal clefts ; but that I regard 

 the increase in their number as a secondary change which has 

 occurred in both lines after their genealogical paths had diverged. 



It does not seem necessary to defend the thesis that the number 

 of gill-slits in the ascidians is the result of secondary multiplication. 



