DEGENERATION OF UNUSED ORGANS. 347 



these influences to which two organisms living in association are 

 exposed, there are yet others which exhibit their results in the 

 degeneration of the organs of true parasites. Most parasites 

 dispense entirely with many organs indispensable to the 

 existence of free-living species ; or else they possess them in so 

 undeveloped a state that they appear almost incapable of ful- 

 filling their functions. They are then called rudimentary 

 organs. It is usually said that this extinction of organs which 

 in other cases are so important, is the result of the parasite's 

 mode of life ; but this is only a brief way of stating the fact 

 that such extreme degeneration has been observed to any great 

 extent only in those true parasites which actually feed on the 

 juices of their host. The statement is in no way an explana- 

 tion of the phenomenon itself, and up to the present time we 

 are absolutely unable to assign with any certainty the causes 

 by which, in any individual case, an organ may have been 

 affected in such a way as to reduce it to the rudimentary condi- 

 tion or to cause it to disappear altogether. It is self-evident 

 that all sedentary parasites, or such as live in the organs of 

 other animals, must lead a free life while young, in order to 

 secure the perpetuation of the species by seeking a new host. 

 In accordance with this, we see that all those parasites with the 

 history of whose development we are acquainted, go through a 

 stage of free larva-existence during which the larva? live under 

 the same conditions as other independent creatures. Such a 

 free existence is, of course, impossible without organs of locomo- 

 tion, such as legs, fins, and so forth ; these again would be use- 

 less if they were not under the control of volition and its 

 auxiliaries, the organs of sensation ; all these organs must be 

 supplied with nourishment, which the free-swimming larva? 

 could not obtain unless they had grasping organs to seize their 

 prey ; and finally, they could not digest the food thus obtained 

 if they were not endowed with organs proper for that function. 

 Vessels or other contrivances must be present in them which 

 may convey the food-juices to the remotest part of the body ; 

 other internal parts must fulfil the task of carrying away all 

 the products of that decomposition of the food-constituents 

 which results from the process of combustion carried on in the 



