THE ANIMALS OF THE FRESHWATER/ AQUARIUM TOI 



the tadpole is much nearer a fish than is a frog. Not only has 

 it gills like a fish, but it has a heart like a fish, it has unpaired 

 fins like a fish, and so on. When the young of an animal occu- 

 pies the same environment as the presumed ancestors of that 

 animal, we frequently find that it is much nearer in structure 

 to this ancestor than the adult form. Many examples of this 

 may be quoted. The young of the shore-crab (p. 145) lives in 

 mid-water and not on the bottom, it is much more like the crab's 

 ancestors than it is like a crab, the caterpillar is more like the 

 supposed ancestors of insects than is the butterfly, and so on. 

 This is what is called the Recapitulation Theory, the view that 

 when the young of an animal is very unlike the adult, the ancestors 

 of the adult are to be sought among the forms which resemble 

 the young rather than among those which resemble the adult. 



The Recapitulation Theory, also known as Von Baer's law, 

 was formulated in the early days of the Evolution Theory, and 

 it was regarded as being, what it undoubtedly is, an extra- 

 ordinarily illuminating interpretation of facts which are otherwise 

 a complete puzzle. It has been since pointed out, however, a 

 fact easy to observe for oneself, that the recapitulation, the 

 repetition of the ancestral characters, is never complete. The 

 tadpole is fish-like, but it is not a fish it has no scales, no paired 

 fins, and so on. In other words, it possesses only those fish-like 

 characters which are necessary to it during its short aquatic life. 

 If the frog did not lay its eggs in water, as happens with some 

 Amphibians, we should expect to find, what we do then find, 

 that the fish-like characters would be reduced in number. In 

 other words, they are present, not, as some of the early evolutionists 

 seemed disposed to think, to give us information about the 

 ancestors of the particular form, but because they are necessary 

 to the life of the animal. 



The development of the eggs of the toad does not differ very 

 notably from that of the frogs. On the other hand, newt tadpoles 

 differ in several respects, and should be reared also, if circumstances 

 permit. 



Neither the common newt (Triton molge) nor the larger and 

 handsomer crested newt (T. cristatus) will thrive in water except 

 at the breeding season, so that neither comes strictly within the 



