396 APPENDIX, 
along the long rudder-tail, a larval organ, which is cast off 
in later transformation. Yet there still exist some very smal] 
ascidie (Appendicularia) which do not become transformed 
and attached, but which through life swim about freely im 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 of 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 pedigrce of 
man beyond the amphioxus into the many-branching tribe of 
‘“‘invertebrate’” worms, from which all 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. 
Pirate XIV. (Between pages 206 and 207, Vol. IL.) 
Monophyletic, 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 paleontological periods of the earth’s 
history. (Compare Chapter XX. vol. ii. p. 192.) The horizontal 
