APPENDIX. 397 
lines indicate the periods (mentioned in vol. ii. p. 14) of the organic 
history of the earth during which the deposition of the strata con- 
taining fossils took place. The vertical lines separate the classes 
and sub-classes of vertebrata from one another. The tree-shaped 
and branching lines, by their greater or lesser number and thick- 
ness, indicate the approximate degree of development, variety, and 
perfection, which each class probably attained in each geological 
period. In those classes which, on account of the soft nature of 
their bodies, could not leave any fossil remains (which is especially 
the case with Prochordata, Acrania, Monorrhina, and Dipneusta) 
the course of development is hypothetically suggested on the 
ground of arguments derived from the three records of creation 
—comparative anatomy, ontogeny, and paleontology. The 
most important starting-points for the hypothetical completion 
of the paleontological gaps are here, as in all cases, furnished 
by the fundamental law of biogeny, which asserts the inner causal- 
nexus existing between ontogeny and phylogeny. (Compare vol. i. 
p- 310, and vol. ii. p. 200; also Plates VITI.—XIII.) In all cases 
we have to regard the individual development (determined by the 
laws of Inheritance but modified by the laws of Adaptation) as 
short and quick repetitions of the paleontological development 
of the tribe. This proposition is the “‘ceterum censeo” of our 
theory of development. 
The statements of the first appearance, or the period of the 
origin of the individual classes and sub-classes of vertebrate 
animals (apart from the hypothetical filling in mentioned just 
now), are taken as strictly as possible from paleontological 
facts. It must, however, be observed, that in reality the origin 
of most of the groups probably took place one or two periods 
earlier than fossils now indicate. In this I agree with Huxley’s 
views; but on Plates V. and XIV. I have disregarded this con- 
sideration in order not to go too far from paleontological facts. 
The numbers signify as follows (compare also Chapter XX. and 
vol. ii. pp. 204, 206) :—1. Animal Monera; 2. Animal Amcebe; 
3. Community of Amcebe (Synameebe); 4. Ciliated Infusoria 
without mouths; 5. Ciliated Infusoria with mouths; 6, Gliding 
