viii EVOLUTION OF TO-DAY. 



the Silurian Diversity of Silurian life Recent discoveries 

 of connecting links Geological position of these links De- 

 velopment of the brain of mammals Paleobotony Sum- 

 mary 



IV. 



EMBRYOLOGY 120 



Importance of embryology Embryology a repetition of 

 past history (i) Truth of this assumption, as shown by com- 

 parison of embryology with paleontology ; with a hypothet- 

 ical history drawn from classification Contradictions arising 

 and their explanation Result of previous considerations 

 Embryology as an assistance in classifying animals. (2) Sig- 

 nificance of the parallel. (3) Application of the principle 

 Difficulties to be overcome in drawing a history from em- 

 bryology Abundance of hypothetical stages necessary 

 Embryological history of animals in general Early diver- 

 gence of types from a common point Summary. 



V. 



GEOGRAPHICAL DISTRIBUTION . . . .165 

 Complexity of the factors Animals not distributed accord- 

 ing to climate Laws of distribution which should follow if 

 the descent theory is true Relation of the present to the 

 past ZoSlogical regions Present distribution of species, 

 genera, families, and orders Do barriers limit the areas of 

 distribution ? Oceanic islands Bermudas Galapagos 

 islands St. Helena Summary. 



VI. 



DARWIN'S EXPLANATION OF EVOLUTION . . 203 

 Summary of the evidence Darwinism Incompleteness of 

 Darwinism Difficulties from the slowness of the modifica- 

 tion ; from the minuteness of changes Transitional stages 

 wanting Specific characters not always useful Ornamental 

 structures Tendency toward elimination by crossing De- 

 velopment of organs by small steps Similarity in indepen- 

 dently acquired organs Mimicry Sterile insects Sum- 

 mary. 



