28 CHAPTERS ON EVOLUTION. 



structure, in the possession of the curious " marsupial bones," in the 

 general arrangement and even special form of internal organs, and 

 in the peculiar shape of the lower jaw, but also in the matter of 

 the foot structure. Very striking is it to observe the prevalence of 

 the one type in the feet of this varied assortment of quadrupeds. 

 " How curious it is," says Mr. Darwin, " that the hind feet of the 

 kangaroo, which are so well fitted for bounding over the open plains 

 those of the climbing, leaf-eating koala, equally well fitted for 

 grasping the branches of trees those of the ground-dwelling, insect- 

 or root-eating bandicoots and those of some other Australian mar- 

 supials should all be constructed on the same extraordinary type, 

 namely, with the bones of the second and third digits extremely 

 slender and enveloped within the same skin, so that they appear like 

 a single toe furnished with two claws ! Notwithstanding this simi- 

 larity of pattern, it is obvious that the hind feet of these several 

 animals are used for as widely different purposes as it is possible to 

 conceive. The case is rendered all the more striking by the Ameri- 

 can opossums, which follow nearly the same habits of life, having feet 

 constructed on the ordinary plan." 



The science of structure thus settles the questions which natur- 

 ally arise respecting the relationships of the kangaroo, by uniting it, 

 in classification, with those forms which truly resemble it in structure. 

 So also with its physiology. The second question, " How does it live? " 

 would be answered in an exact fashion by the investigation of the 

 life-processes of the animal, and by the knowledge which physiology 

 would bring to bear upon the manner in which kangaroo-existence 

 is divided, like that of all other animals, between supporting its 

 frame, increasing its race, and maintaining relations with the world 

 around. 



The question, " Where is it found ? " involves in its reply, in the 

 case of the kangaroo, a large number of highly interesting and instruc- 

 tive considerations. Kangaroos are found in Australia and adjacent 

 islands alone. Why are they limited to this region of the earth's 

 surface ? and why, to put this question more generally, has Australia 

 no native quadrupeds other than these marsupials and their near rela- 

 tions ? for it need hardly be added that the horse, cow, sheep, and 

 allied animals are all of recent introduction by the hand of enter- 

 prising, colonising man. Looking at a zoological map of the world a 

 chart prepared solely with reference to the distribution of animal life 

 we should observe that the animals peculiar to Australia stop short on 

 one side of a line called " Wallace's Line," which passes in one part of 

 its course between the little islands of Bali and Lombok in the Eastern 

 Archipelago. The Straits of Lombok are about fifteen miles in width, yet 

 that narrow sea divides the land of marsupials Australia and adjacent 

 islands from other lands and islands in which no marsupials are found. 



