CHAPTER VII. 



THE COMMON FROG. 



Class, Amphibia. Order, Ecaudata. Family, Rdnidce. Genus, 

 Rana. Species, temporaria. 



Our common frog is a very important little animal. Not only 

 has it been of immense service to science, and especially to 

 medical science, by the numbers of experiments practised upon 

 it, thus affording information of vast utility to the human race 

 (in which case only are such experiments justifiable), but, as will 

 be remembered, in leading to the discovery of galvanism. The 

 frog is, besides, the highest of the Amphibians ; that is, in its 

 anatomical structure it approaches more nearly to the higher 

 vertebrates than any other of its class. By the tables given in the 

 Introduction it was seen that in each successive sub-kingdom 

 there is a gradual advance — what we may call an improvement of 

 organism — and additions to organs. Beginning with the mere cell, 

 by degrees a mouth, limbs, eyes, heart, brain, external coverings, 

 and so on are developed or perfected, until the back-boned animals 

 are reached ; and in these the progressive stages are still observ- 

 able, so that the terms " higher " or " lower " in development are 

 understood to mean their place in this graduated scale. In some 

 of the intermediate forms — called the lowest vertebrates — the 

 backbone is never perfected, but remains a mere cartilage, never 

 hardening into bone: in these cases the noto-chord (see p. 45) is 

 largely or entirely persistent, as in the Tunicates, alluded to- in 

 the Introduction ; an intermediate group offering an example 

 of this advance from the lowjer to the higher creatures. They 

 have intruded' themselves among our English reptiles for reasons 

 there given. But even since this Introduction was written those 

 researches of which I spoke have resulted in the discovery of 

 a third eye in fishes as well as lizards ; and " a rudimentary trace 

 of it in all other groups of vertebrates including the Amphibia." 

 Professor J. Beard, of the University of Freiburg, has, among 

 others, been investigating these " Parietal " eyes in fishes, etc. 

 He says, "Something that admitted of comparison could be 



