XI J CONTENTS. 



inces, 113. Normal adaptation to conditions of environment Specific 

 centres of distribution and varieties, 114. The distinctness of the flora 

 and fauna of distinct provinces The various classifications of natural- 

 history provinces, 115. Marine organisms particularly important to 

 the paleontologist Haeckel's classification of the marine conditions 

 of life Walther's further analysis of conditions of environment, 116. 

 Relations of organisms to time and to environment equally signifi- 

 cant, 117. An explanation required for succession of species as well 

 as for adjustment of species Evolution and adaptation both observed 

 facts, 118. Ancestry and environment as causes of evolution Differ- 

 ences of opinion respecting interpretations, not facts Introduction of 

 causation into the discussion, 119. Ancestry and Environment in 

 relation to the beginning of each individual Definition of the terms 

 "Ancestry" and "Conditions of Environment," 120. Two factors 

 producing the effects of evolution Three views possible First cause 

 of some sort essential to any complete theory of evolution, 121. 

 Edward Forbes on origin of species and centres of creation Reality 

 of specific centres not questioned; the fact variously interpreted, 122. 

 Representative species, common descent, and migration of species 

 Darwin did not deny the facts, but explained them differently from 

 Forbes Forbes' explanation of the origin of species, 123. The mean- 

 ing of evolution by descent Distinction between Evolution and De- 

 velopment, 124. Immutability or mutability of species, 125. Muta- 

 bility of species the central thought in the new theory of the origin of 

 species Two extremes of opinion regarding the mode of origin of 

 species by evolution, 126. An unknown cause assumed to explain 

 origins by both Forbes and Lamarck, 127. Conclusions, 128. 



CHAPTER VII. 



GEOGRAPHICAL DISTRIBUTION: SPECIAL CONSIDERATION: THE ADJUST- 

 MENT OF ORGANISMS TO ENVIRONMENT. 



Resume, 129. Gastropoda illustrate the law of relationship between 

 organisms and environment Meaning of the classification of organ- 

 isms, 130. Distinguishing characters of the class Gastropoda, 131. 

 Zones of environment in which Gastropods are distributed, 132. 

 Reasons for selecting the Gastropods Peculiarity of the divisions of 

 the Gastropods as to range of adaptation, 133. Mode of existence of 

 the Glossophora, 135. The zonal distribution of the Ctenobranchina, 

 136. Genera of the Ctenobranchina characteristic of the several 

 bathymetric zones, 137. Evidence of the adjustment of the morpho- 

 logical characters to environment Law of the adjustment of organ- 

 isms to conditions of environment, 138. Summary, 139. Relation 

 between zonal adaptation and geographical range, 140. Families 

 whose genera have a very wide range of adaptation, and restricted 

 adjustment only among the species Great difference in the closeness 

 of adjustment of the characters of different taxonomic rank, 142. 

 Species generally closely adjusted to particular conditions Fresh- 

 water families; restriction in their distribution Two closely allied 

 families separated in their distribution, 143. Table of the Geological 

 range of the families Strombidse and Chenopodidae, 144. Relation of 



