I.] INTRODUCTORY. 



most interesting theory, in relation to natural science, 

 which has been promulgated during the present century. 

 Eemarkable indeed is the way in which it groups to- 

 gether so vast and varied a series of biological ^ facts, and 

 even paradoxes, which it appears more or less clearly to 

 explain, as the following instances will show. 



By this theory of " Natural Selection," light is thrown 

 on the more singular facts relating to the geographical 

 distribution of animals and plants ; for example, on the 

 resemblance between the past and present inhabitants of 

 different parts of the earth's surface. Thus in Australia 

 remains have been found of creatures closelv allied to 

 kangaroos and other kinds of pouched beasts, which in 

 the present day exist nowhere but in the Australian 

 region. Similarly in South America, and nowhere else, 

 are found sloths and armadillos, and in that same part 

 of the world have been discovered bones- of animals 

 different indeed from existing sloths and armadillos, but 

 much more nearly related to them than to any other kinds 

 whatever. Such coincidences between the existing and 

 antecedent geographical distribution of forms are numerous. 

 Again, " Natural Selection " serves to explain the cir- 

 cumstance that often in adjacent islands we find animals 

 closely resembling, and appearing to represent, each other ; 

 while if certain of these islands show signs (by depth of 

 surrounding sea or what not) of more ancient separation, 

 the animals inhabiting them exhibit a corresponding 

 divergence.^ The explanation consists in representing 

 the forms inhabiting the islands as being the modified 



1 Biology is the science of life. It contains z'oology, or the science of 

 animals, and botany, or that of plants. 



2 For very interesting examples, see ]Mr, Wallace's " Malay Archi])elago. " 



