X ON THE STUDY OF BIOLOGY 275 



swers for the other. They carr}' their microscopic 

 inquiries in the case of the dog as far as they 

 can, and they find that his body is resolvable into 

 the same elements as those of the man. More- 

 over, thev trace back the do2;'s and the man's devel- 

 opment, and they find that, at a certain stage 

 of their existence, the two creatures are not dis- 

 tinguishable the one from the other; they find 

 that the dog and his kind have a certain distri- 

 bution over the surface of the world, comparable 

 in its way to the distribution of the human species. 

 AYhat is true of the doo; thev tell us is true of 

 all the higher animals; and they assert that they 

 can lay down a common plan for the whole of 

 these creatures, and regard the man and the dog, 

 the horse and the ox as minor modifications of 

 one orpeat fundamental unitv. ^loreover, the in- 

 vestigations of the last three-quarters of .a century 

 have proved, they tell us, that similar incjuiries, 

 carried out through all the difl'erent kinds of ani- 

 mals which are met with in nature, will lead us, 

 not in one straight series, but by many roads, step 

 by step, gradation by gradation, from man, at the 

 summit, to specks of animated jelly at the bottom 

 of the series. So that the idea of Leibnitz, and 

 of Bonnet, that animals form a great scale of 

 being, in which there are a series of gradations 

 from the most complicated form to the lowest and 

 simplest; that idea, though not exactly in the 

 form in which it was propounded by those phi- 



