1 6 CONTENTS 



parasites from free animals. Organic changes in parasites. 

 How parasites are conveyed. Exchange of hosts. Life of 

 the chief parasites : Trichinae, maw-worms, dochmius, 

 tape-worms, etc. Danger of fins. Development of the 

 liver-distoma. Friendships of animals. Symbioses - 262 



CHAPTER IX. PROTOZOA. 



The animal built up of cells. Principle of division of labour. 

 The greater the division of labour, the higher the animal's 

 organisation. Multicellular and unicellular animals. The 

 protozoa, their form and reproduction. Structure of an 

 animal in its development from the ovum onwards from 

 rudimentary parts. Origin of the germ-cells. Outlines 

 of a theory of heredity. Amphimixis of the protozoa. 

 Origin of sexual reproduction. Formation of seed and 

 ova. Continuity of the germ-cells. Are any animals 

 immortal ? Death is not common to all animals. Origin 

 of death. Permanent and temporary life. Has life come 

 from the stars? Origin of life on the earth, spontaneous 

 generation. How it is to be conceived. The first develop- 

 ment of living matter. Formation and significance of the 

 cell - nucleus. Significance of the fundamental parts. 

 Relation of the rudimentary parts in amphimixis. When 

 life is extinguished - - 288 



CHAPTER X. EXTENSIONS OF THE PRINCIPLE OF 



SELECTION AND OTHER INVOLUNTARY THEORIES 

 Why there are numerous species to-day. Isolation facilitates 

 the divergence of species. Modification of isolated animals. 

 Movement and alteration of animals. Are species formed 

 by isolation even without the aid of natural selection? 

 Definite variations. Germinal selection. Changes in the 

 nutrition of the embryo are the foundation of variations. 

 Do useless organs disappear through germinal selection ? 

 Refutation of germinal selection. Effect of external in- 

 fluences as modifying principle. Orthogenesis. Rejection 

 of same. The mutation-theory. Variations and mutations. 

 Do variations proceed indefinitely? Is there a formative 

 energy in organisms? Mechanicism and vitalism. How 

 are the form and purposiveness of organisms explained? 

 What is to be understood by chance. Absence of purpose 

 in living things. The will to live, the instinct of self- 

 preservation - -324 



