304 TADPOLES OF THE SEA 



described a century ago by the poet Chamisso, who 

 wrote the story of Peter Schlemihl, the man who sold his 

 shadow to the devil ! The term " alternation of genera- 

 tions " was introduced to describe the extraordinary state 

 of affairs discovered by Chamisso in the Salpa?. We now 

 know that what occurs in the Salpae is only one striking 

 example of a proceeding which is very frequent in a 

 variety of kinds of both animals and plants. The " alter- 

 nation " of generations, differently produced and of 

 different appearance, is known to occur in many parasitic 

 worms, in many insects, such as the gall-flies and plant- 

 lice, and in the coralline polyps, whilst it is universal in 

 mosses, ferns, and (in a hidden inconspicuous way) in all 

 the flowering plants. (See Chapters VII and VIII.) 



It is practically certain that the whole group of 

 " Ascidians " or " Tunicata," which in extreme cases show 

 us mat-like and multiple budded forms like vegetables 

 rather than animals, are the much simplified, specialised 

 descendants of active, highly developed vertebrate fish-like 

 ancestors. Fifty years ago the view was very general 

 among naturalists that the simpler animals are necessarily 

 the more primitive. In earlier times the notion existed 

 that living things could be naturally arranged in a single 

 series, leading step by step from the simplest forms 

 through more complex forms to the highest and most 

 perfect. This single series was called the " scala naturae " 

 or " ladder of life." But the great French zoologist, Cuvier, 

 at the beginning of the nineteenth century, showed that 

 there are at least four such ladders diverging from one 

 another and quite distinct from one another the four 

 " branches " he called them. They were, according to 

 him, the radiated animals (star-fishes, corals, etc.), the 

 molluscous animals (snails, clams, cuttle-fish), the articu- 

 lated animals (crustaceans, insects, spiders and scorpions), 



