26 CHAPTERS ON EVOLUTION. 



when he endeavours to trace through the mazes and byways of evolu- 

 tion the manner in which the living world and all that is therein com- 

 prised has been formed, moulded, and perfected as we now find it 

 If, therefore, as we shall hereafter see, there are means and ways, 

 clues and traces, to be found in nature for the study of the method 

 through which living beings have come to assume their existing 

 order, it were but folly to deny our right to utilise such means to the 

 full, and to extend that knowledge, the increase of which Bacon 

 wisely declared tended to the relief of man's estate. 



" ^Etiology? or the " science of causes," thus supplies us with 

 the reply to the last of the four queries which concern the nature of 

 animals and plants. In itself, this branch of inquiry connects the 

 other three departments. It utilises the knowledge which structure, 

 physiology, and distribution collect and systematise. It supplies the 

 natural termination to all inquiries respecting the history of living 

 beings. Since we believe that the causes which have wrought out 

 the existing order of nature have left traces of their operation in the 

 living universe ; which traces, like the silver thread running through 

 the many-coloured pattern, serve to link together the interests and to 

 show the harmonies which underlie the varied warp and woof of life. 



To fix these methods of biological study the more firmly upon our 

 minds, we may select, as the subject of a brief exposition, the natural 

 history of a kangaroo an animal form sufficiently distinct and spe- 

 cialised to render the details of its biological study a matter of easy com- 

 prehension. No animal form is more familiar as a being foreign to our 

 own country than the kangaroo ; and its history, like that of every other 

 living being, familiar or otherwise, must be investigated along the lines 

 we have just laid down. The question " What is it ?" is answered by 

 morphology ; and a large number of very interesting replies would be 

 found amongst the answers to the questions of the science of structure. 

 We should thus be informed, as a primary fact of kangaroo history, 

 that it is a Vertebrate, or " backboned" animal ; that it agrees in the 

 general type of its body with all fishes, reptiles, birds, and quadrupeds ; 

 and we should, moreover, speedily discover by even a cursory ana- 

 tomical examination that it belongs to the quadruped class, and 

 presents essentially the same general characteristics which all mammals 

 or quadrupeds, from the whale upwards to the lion, dog, rat, sheep, ape, 

 and man, agree in possessing. But the more personal history of our 

 kangaroo would show wide differences in structure from the organisa- 

 tion of ordinary quadrupeds. We should be struck by the low type 

 of its brain, as compared with the brain of ordinary quadrupeds. 

 We should note two curious bones unknown in common animals, 

 and which arise from the front of the kangaroo's haunch-bones. These 

 are the so-called " marsupial bones," on which the " pouch " these 

 animals possess is supported. In connection with this fact of kangaroo- 



