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 frora the three records of creation 

 — comparative anatomy, ontogeny, and palaeontology. The 

 most important starting-points for the hypothetical completion 

 of the pala3ontological gaps are here, as in all cases, furnished 

 by the fundamental law of biogeny, which asserts the inner causal- 

 nexus existing between ontogeny and fliylogeny . (Compare vol. i. 

 p. 310, and vol. ii. p. 200 ; also Plates VIII.— XIII.) In all cases 

 we have tp regard the individual development (determined by the 

 laws of Inheritance but modified by the laws of Adaptation) as 

 short and quick repetitions of the palseontological 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 jiist 

 now), are taken as strictly as possible from palseontological 

 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 palgeontological facts. 



The numbers signify as follows (compare also Chapter XX. and 

 vol. ii. pp. 204, 206) : — 1. Animal Monera ; 2. Animal Amoebas ; 

 3. Community of Amoebae (Synamoebae) ; 4. Ciliated Infusoria 

 without mouths ; 5. Ciliated Infusoria with mouths ; 6. Gliding 



