356 CHAPTERS ON EVOLUTION. 



parents. Received, along with food, into the body of the hare or 

 rabbit, the larval being liberates itself. From the rabbit's digestive 

 system it bores its way through the tissues to the liver, thus reminding 

 one strongly of the similar migrations of the embryo -tape worm. In 

 the liver, further changes ensue. Frequent mountings become the order 

 of the day; and at length they assume a worm-like aspect, and remain 

 thus, still imperfect, until, by transference to the body of dog, wolf, 

 or sheep, and by passage to the frontal sinuses, they acquire perfection 

 of their life-functions. If the history of these beings teaches us 

 anything concerning their past, it points to a free and active state 

 as their original condition, and to the probable acquirement, first, of 

 a lodgment in the digestive system of one animal as a relatively simple 

 parasite, and secondly, of a further modification of habit transferring 

 at once its perfection and completed degradation to the forehead- 

 cavities of a second host. 



But the conditions which make for the degeneracy of an animal 

 are, as we have seen in the case of the barnacles, not always associated 

 with a parasitic habit. Mere fixation, as we have 

 observed, secures the disappearance of useless 

 organs, such as organs of motion and sense- 

 organs, which, being possessed by the young 

 form, clearly indicate that the ancestry of the 

 animals in question has at any rate been capable 

 of leading to better things than the descendants 

 represent in their existent persons. The sea-squirts, 

 or ascidians, besides serving as a text for the 

 derivation of vertebrates, and for abnormal ways 

 in the animal chemistry which imitates the plant's 

 work, have been selected as fruitful objects of 

 discussion by those biologists who find in the 

 idea of degeneration an explanation of knotty 

 points in natural history. For the same voice 

 FIG. 256. SEA-SQUIRT, that proclaims the fact that a sea- squirt which is 

 a mere rooted bag with a double neck (Fig. 256) begins life as a 

 free- swimming, tadpole-like larva (Fig. 257, 5), tells us in the same 

 breath that there must have been retrogression and degeneration 

 from an active condition to produce the sac-like adult state. The 

 assertion that the youthful sea-squirt, moreover, possesses first a 

 rod-like body called \htnotocJwrd (Fig. 257, n) only found besides 

 in the young of Vertebrate animals, is also to be taken as implying 

 the superiority of ascidian infancy to sea-squirt maturity. And 

 when it is added that the elderly squirt wants the sense-organs and 

 nervous cord which the larva possesses, if may well be argued that 

 sheer degeneracy of habit and structure can alone account for the 

 sweeping transformations which mark the phases of ascidian life- 



