TELL-TALE YOUNG STAGES 305 



and the vertebrated animals (fish, reptiles, birds and 

 mammals). Still, it was considered that in each great 

 branch the animals of simplest appearance and least 

 elaborate build were essentially more " primitive " or 

 "lower" that is, lower in the scale of nature than the 

 more highly elaborated. And when Darwin's doctrine of 

 the origin of the living kinds of animals and plants from 

 simpler ancestors by gradual development in long ages of 

 time was established, it was naturally held that the 

 simpler forms were survivals of the earlier ancestral forms 

 of the more complicated kinds, or at any rate showed 

 more or less clearly what the ancestral forms were like. 



The recognition of so-called " degeneration," or, to use 

 an Irish "bull," "progress backwards" or simplification, as 

 a sequel, in the ancestral history of many animals, to 

 previous high development and complication of structure, 

 has rendered it necessary to take account of the fact that 

 any simple kind of animal which we now find living, may 

 (not " very probably," but still " possibly ") be the simpli- 

 fied descendant of much more highly developed and com- 

 plicated ancestors. Anton Dohrn, who founded the marine 

 laboratory at Naples, was the first to insist on this, and 

 I gave an evening lecture on the subject at the meeting 

 of the British Association in 1879, published subse- 

 quently under the title " Degeneration a chapter in 

 Darwinism." A very important fact bearing on this 

 matter is that, although such simplified animals often have 

 young stages (such as the Ascidian's tadpole), which " give 

 away " the adults and tell the story of their former high 

 condition and their true blood relationship to more elab- 

 orated creatures, yet often such tell-tale young stages do 

 not occur. There are many Ascidians which have no 

 tadpole stage, but grow from the egg straight off into the 

 sac-like adult condition,without showing a trace of the tad- 

 pole structure ! Supposing (as might well have happened) 



20 



