252 THE PERMANENTLY 



explain the second branch of the difficulty. You 

 will recollect that I have said (page 

 158) that the higher animals of a group 

 pass through certain forms in their de- 

 velopment which correspond to the per- A 

 manent adult condition of the lower 

 animals of that same group ; e. g., the 

 Fjo . B 140 Rabbit passes from that degree of or- Fi( 150 

 ganic simplicity which corresponds to. that of a fish, to a higher 

 state which resembles that of a reptile, and then onward to that 

 phase which is typical of the organization of a bird, and finally it 

 assumes its adult condition. A Butterfly, one of the highest of 

 the group of Articulata, in its youngest stages is a grub or worm, 

 so-called, whose numerously jointed body and frequently re- 

 peated internal organs recall the many-jointed, true worms, such 

 as the earth-worm, and marine worms, (p. 80, fig. 43,) but more 

 strictly correspond to those of the earwigs, and the thousand-legs, 

 or ringed ivorms, so common under sticks and stones on the 

 ground. This idea was first put forth by Von Baer, thirty-six 

 years ago, in 1828, (Uber Entwickelungsgeschichte, &c., Theile 

 i. p. 230,) and, taking the cue from him, other observers have 

 traced its prevalence throughout the animal kingdom, not only 

 among the living, but also through the numerous groups of 

 extinct fossil animals of past ages. 



From this you may judge that all the members of a group do 

 not come to the same degree of perfection, but that some remain 

 in what corresponds to the embryonic stages of the higher ani- 

 mals of that group. Now from this you may also infer that we 

 would have the same difficulty with these permanently embry- 



Fig. 149. Pneumodermon violaceum. D. Orb. Natural size, 0.15'", i, e., J^ 

 of a line long. A worm-like stage of development. A, the head ; B, the tail. 

 The body is encircled by three groups of vibratile cilia. From Gegenbaur. 



Fig. 150. Pneumodermon violaceum. D. Orb. 3 diam. An adult Pteropo- 

 dous Molluscan which swims in the open sea. A, the head ; b, the main part of 

 the body; c, the tail; _/", the pair of feelers; y 1 , the suckers of f; to, the fins 

 which are used like wings. From Woodward. 



