VI TABLE OF CONTENTS. 



PAGK 



Foundation of Modern Zoology 21 



The Division of the Radiata = 21 



C. Th. v. Siebold 21 



Leuckart 22 



The Present System 22 



History of the Theory of Descent 23 



Importance of the Subject 23 



The Nature of Species 23 



Ray's Conception of Species 23 



The "Cataclysm Theory" 24 



Objections to the Cataclysm Theory 25 



Darwin's Predecessors 26 



Lamarck 26 



Evolution vs. Creation 27 



Lyell 27 



Darwin 28 



Post-Darwinian Writers 29 



Darwin's Theory of the Origin of Species 30 



The Problem Stated 3 



Morphological Characters 30 



Physiological Characters 32 



A. Crossing of Species and Varieties 32 



B. The Intercrossing of Species 33 



C. The Fertility of Hybrids and Mongrels 34 



D. Inbreeding 35 



Conditions Governing Fertility in Sexual Reproduction . . 35 



Difficulties in Systemization 35 



Change of Varieties into Species 36 



Species may be Related to Each Other in Unequal Degrees 36 



Phylogeny 36 



Spontaneous Generation 37 



Variability Not Proven to be a Universal Principle ... 38 



Proofs of Phylogeny 38 



(1) Proofs from Systemization 38 



(2) Paleontological Demonstration 39 



Examples of Paleontological Proof 39 



(3) Morphological Proofs 40 



Fundamental Law of Biogenesis 41 



Examples of the Application of this Law 44 



Study of the Distribution of Animals 49 



Instances of Proofs 50 



Causal Foundation of the Theory of Descent 51 



Artificial Selection 5- 



Factors of Evolution in Breeding 53 



Factors of Evolution in Nature 53 



Natural Selection- Struggle for Existence 53 



