THE SIGNIFICANCE OF THE TADPOLE STAGE. 71 



ations of the Ascidian pharynx, both larval and adult, with the gill 

 clefts of the Lancelet specially, and of fishes generally. 



Huxley and Haeckel warmly espoused this view so carefully and 

 logically put forward, and now it reigns as the orthodox opinion. 

 Prof. W. K. Brooks, who has within the last few weeks published 

 an extremely valuable monograph upon the Salpce, a group of much 

 modified and altered pelagic Tunicates,* after a fresh and independent 

 investigation from new stand-points and with many new facts to put 

 into witness, fully endorses the same view, and states his belief 

 that a simple pelagic form, of which Appendicularia is the nearest 

 living representative, is the common ancestor of all the Chordata, i.e. 

 alike of the Tunicata as of the Lancelet and all the Yertebrata. 



Of course such theory can never be proved with mathematical 

 certainty. It is at best a plausible probability, the most probable 

 explanation of the mode of descent of the higher animals we at 

 present know of. But there are, and have been, capable investigators 

 who cannot bring themselves to accept it Thus Prof. Giard, who as 

 the first occupant of the chair of Darwinism at the Sorbonne, cannot 

 be considered in any way a scientific reactionary, in 1872, after 

 elaborate investigation of the Compound Ascidians, refused to endorse 

 this opinion. He then could see nought but a convergence or coinci- 

 dence in general form between Ascidian larvas and lowly vertebrates, 

 due he believed to a similar mode of life, i.e. free-swimming. He thus 

 considered the adult as the true or fundamental form of the Tunicate 

 group, and the larvae not as a reminiscence of ancestral freedom, but 

 as a larval form that for its special and temporary needs, assumed 

 through like requirements of mechanical stresses, due to the adoption 

 of a similar mode of progression, swimming, a form akin to that of 

 a lowly vertebrate. 



If we want to go astray in reasoning, it is often easy enough 

 with the exercise of a little ingenuity or else of some obtuseness to 

 sink deep in the mire of false deduction. Anyone not thoroughly 

 master of an intricate subject may easily mistake cause for effect, 

 and conversely, and I remember once when considering this Ascidian 

 problem, thinking what a grand opportunity exists in the life-history 

 of Appendicularians for such an one to go astray. Among these tiny 

 tailed swimmers, the habit exists, at certain times, of forming a 

 gelatinous envelope in which the animal lies enveloped for a short 

 time the " Haus " this has been termed ; and it is obviously 

 homologous to the test of the fixed Ascidians. Now a very 

 probable line of descent of the latter is from Appendicularians 

 that have taken this " haus " stage as a permanent adult condition 



* " The Genus Salpa," Baltimore, 1893. 



