MICROSCOPICAL STUDIES. 107 



not ripen at the same time. The young produced are the tadpole 

 larvae we have above described. 



Only within the last 28 years have the Tunicates bulked with 

 large importance in scientific ken. Until 1866, when the Russian 

 naturalist Kowalewsky published his famous researches and specu- 

 lations upon the embryology of the group, they occupied a position 

 of comparative isolation, with ill-understood affinities, and were 

 generally treated as aberrant forms of little importance. Some were 

 for placing them among the Mollusca, largely upon the count of their 

 acephalous (headless) condition ; others from the peculiar looped 

 form of the alimentary canal would have that they were peculiarly 

 developed relatives of the Polyzoa. The lowly development of the 

 venous system favoured alike either of these views. 



But Kowalewsky's researches altered all that, and gave the 

 Tunicates a status of great importance. He pointed out how the 

 adult Appendiculariae and the tadpole larvse of the fixed Ascidians 

 bear many close resemblances to the lower vertebrates, especially to 

 the Lancelet (Amphioxus), such for instance as the presence in both 

 of a stiff central axis, the notochord, and the occurrence of a central 

 nervous system expanding into a large cerebral vesicle at the anterior 

 end. Finally he pointed out the perfect concordance of the perfor- 

 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 SaljJce, 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 Vertebrata. 



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- 



* " The Genus Salpa," Baltimore, 1893. 



