xii CONTENTS 



THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 

 The Problems of Geographical Distribution ; and the methods of 

 attacking them. The clue to the present condition is afforded 

 by the study of past history. 

 The Geographical Distribution of Camels, Marsupials, and Tapirs. 



LECTURE IV. (Pp. 78-115.) 



THE ARGUMENT FROM EMBRYOLOGY. 



Embryology as a clue to Zoological affinities. The development of an 

 animal often gives us evidence, otherwise unattainable, as to its 

 relations with other animals or groups of animals. Good illus- 

 trations of this are afforded by Ascidians, by Barnacles, and by 

 many groups of Parasitic animals. 



The Recapitulation Theory. The doctrine of Evolution tells us that 

 animals, like men, have pedigrees. The study of Embryology 

 reveals to us this ancestry, because every animal in its develop- 

 ment tends to repeat the history of the race. 



Illustrations of Recapitulation. The development of Flat fish, Crabs, 

 Prawns and Barnacles. 



Embryology and Paleontology. Examples of Recapitulation as seen 

 in Fossils. The Shells of Foraminifera and of Mollusca. The 

 Embryology of Ammonites. The Antlers of Deer. 



Rudimentary or Vestigial Organs : structures which, are present in a 

 condition in which they can be of no use to their possessors. 

 Natural Selection will not account for the formation or 

 perpetuation of such structures ; but the Recapitulation Theory 

 explains these at once, as organs which were of functional value 

 to the ancestors of their present possessors, and which appear 

 in the development of existing forms owing to the tendency to 

 repeat ancestral characters. Examples of Vestigial Organs : 

 their Zoological importance. 



Causes tending to falsify the Ancestral History as preserved in actual 

 development. 



1. The tendency to condensation of the Ancestral History. 



2. The tendency to the omission of Ancestral Stages. The 

 structure of an egg. Germ yolk and food yolk. The causes 

 regulating the number and size' of the eggs produced by 

 different animals. 



3. The tendency to distortion of Ancestral Stages. 



