10 COMPARATIVE ANATOMY. 



development is in the closest connection with the amount of activity. 



But since, when a function ceases or diminishes, atrophy commences, 



we obtain, as a result of the process, rudimentary organs. They 



owe their origin to atrophy. Physiology alone, theu, can give us the 



explanation of the origin of these organs ; and thus again we are 



led to observe the great influence which it exerts on the study of 



Morphology. 



§ 10. 



An organ can be so much changed by the gradual modification 

 of its function that it becomes, from the physiological point of view, 

 anew one, and is then placed in quite another physiological category 

 of organs. This fact is of considerable importance, for it helps to 

 explain the appearance of new organs, and obviates the difficulty 

 raised by the doctrine of evolution — viz. that a new organ cannot at 

 once appear with its function completely developed ; that it there- 

 fore cannot serve the organism in its first stages whilst it is gradually 

 appearing ; and that consequently the cause for its development can 

 never come into operation. Every organ for which this objection has 

 the appearance of justice can be shown to have made its first ap- 

 pearance with a significance differing from its later function. Thus, 

 for example, the lungs of the Vertebrata did not arise simply as a 

 respiratory organ, but had a predecessor among fishes breathing by 

 gills, in the swim-bladder, which at first had no relation to respiration. 

 Even where the lungs first assume the functions of a respiratory 

 organ (Dipnoi, many Amphibia) they are not the sole organ of 

 the kind, but share this function with the gills. The organ is there- 

 fore here caught, as it were, in the stage of conversion into a 

 respiratory organ, and connects the exclusively respiratory lungs 

 with the swim-bladder, which arose as an outgrowth of the enteric 

 tube and was adapted to a hydrostatic function. 



The earlier function of an organ which by adaptation is converted 

 to new uses is generally a lower one, and less important for the 

 organism, in comparison with the new function which is taken on, 

 so that the organ thus rises to a higher grade. In other cases the 

 value of the primary function is less, because it is shared by other 

 similar organs. It is then quantitatively lower, for a share is taken 

 by the other similar organs in discharging the total amount of the 

 function necessary for the full activity of the organism. The 

 atrophy of some of the organs which are of equal value raises the 

 value of those that remain by causing their higher development. 

 To these facts, as to their change of functions, the difference in the 

 classification of organs, accordingly as we make use of a physio- 

 logical or a morphological method, is due. 



