84 ON THE ORIGIN AND [CHAP. 



stance that of insects, have probably come to be what 

 they are, assuming 1 them to have developed under 

 natural laws from simpler organisms. The question 

 is one of great difficulty. It is hardly necessary to 

 say that insects cannot have passed through all the 

 lower forms of animal life, and naturalists do not at 

 present agree as to the actual line of their develop- 

 ment. 



In the case of insects, the gradual course of evo- 

 lution through which the present condition of the 

 group has probably been reached, has been discussed 

 by Mr. Darwin, by Fritz Miiller, Haeckel, Brauer, 

 myself and others. 



In other instances Palaeontology throws much light 

 on this question. Leidy has shown that the milk- 

 teeth of the genus Eqims resemble the permanent 

 teeth of the ancient A nc/iit/iermm, while the milk-teeth 

 of A nchithcrium again approximate to the dental sys- 

 tem of the still earlier Merychippus. Riitimeyer, while 

 calling attention to this interesting observation, adds 

 that the milk-teeth of Equus caballus in the same way, 

 and still more those of E. fossilis, resemble the perma- 

 nent teeth of Hipparion. 



" If we were not acquainted with the horse," says 

 Flower, 1 " we could scarcely conceive of an animal 

 whose only support was the tip of a single toe on 

 each extremity, to say nothing of the singular con- 

 formation of its teeth and other organs. So striking 

 have these characters appeared to many zoologists, 

 that the animals possessing them have been reckoned 

 as an order apart, called Solidungula ; but palaeon- 



1 Journal of the Royal Institution. April 1873. 



