X ON THE STUDY OF BIOLOGY 275 



the one answers for the other. They carry 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. Moreover, they trace back the dog's and 

 the man's development, and they find that, at a 

 certain stage of their existence, the two creatures 

 are not distinguishable- the one from the other; 

 they find that the dog and his kind have a certain 

 distribution over the surface of the world, com- 

 parable in its way to the distribution of the human 

 species. What is true of the dog they 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 modifica- 

 tions of one great fundamental unity. Moreover, 

 the investigations of the last three-quarters of a 

 century have proved, they tell us, that similar 

 inquiries, carried out through all the different kinds 

 of animals 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 philo- 



