39^ APPENDIX, 



along the long rudder-tail, a larval organ, whicli is cast off 

 in later transformation. Tet there still exist some very small 

 ascidisB (Appendicularia) which do not become transformed 

 and attached, but which through life swim about freely in the 

 sea by means of their rudder-tail. 



The ontogenetic facts which are systematically represented on 

 Plate XII. and which were first discovered in 1867, deserve the 

 greatest attention, and, indeed, cannot be too highly estimated. 

 They fill up the gap which, according to the opinion of older zoolo- 

 gists existed between the vertebrate and the so-called " inverte- 

 brate " animals. This gap was universally regarded as so im- 

 portant and so undeniable, that even eminent zoologists, who 

 were not disinclined to adopt the theory of descent, saw in this 

 gap one c'f the chief obstacles against it. Now that the ontogeny 

 of the amphioxus and the ascidia has set this obstacle completely 

 aside, we are for the first time enabled to trace the pedigree of 

 man beyond the amphioxus into the many-branching tribe of 

 "invertebrate " worms, from which aU the other higher animal 

 tribes have originated. 



If our speculative philosophers, instead of occupying them- 

 selves with castles in the air, were to give their thoughts for some 

 years to the facts represented on Plates XII. and XIII., as well 

 as to those on Plates II. and III., they would gain a foundation 

 for true philosophy — for the knowledge of the universe firmly 

 based on experience — which would be sure to influence all 

 regions of thought. These facts of ontogenesis are the in- 

 destructible foundations upon which the monistic philosophy 

 of future times will erect its imperishable system. 



Plate XIV. (Bekveen pages 206 a7id 207, Vol. 11.) 



Monopliyletic, or One-rooted Pedigree of the Vertebrate Animal 

 tribe, representing the hypothesis of the common derivation of 

 all vertebrate animals, and the historical development of their 

 different classes during the palseontological periods of the earth's 

 history. (Compare Chapter XX. vol. ii. p. 192.) The horizontal 



