138 OUR REPTILES. 



ovum, whence the number contained in so many 

 cubic inches is calculated, we cannot comprehend 

 how that number is influenced by the ova lying in 

 pairs or singly. Further, the observation that in 

 one species the line is single, and in the other 

 double, arises probably from regarding the alternate 

 ova in the one chain as a single zigzag line, and not 

 as a double chain with the ova alternately arranged. 

 After adverting to the uses of the tadpoles in the 

 economy of nature, the same gentleman enquires, 

 " What becomes of these myriads of tadpoles ? " 

 and sets about to furnish an answer : 



Few of those vast swarms that blacken the waters in spring 

 with their dusky forms ever reach the perfect frog. Their 

 enemies are many, their means of defence few. They become 

 the prey of larger or more warlike animals than themselves. 

 These constant attacks greatly thin their numbers. Thus by 

 the time they are fit to leave the water, they are, though still 

 somewhat numerous, much less so than at an earlier period of 

 their existence. But having left the waters, they are still 

 exposed to great dangers. They are greedily devoured by the 

 snake, weasel, polecat, and by nearly every species of water- 

 fowl. 



Amongst the creatures who feed largely upon the 

 toad and frog in its larval state are enumerated the 

 larvae of the dragon-flies (Libellula), which are ex- 

 ceedingly destructive ; the larvae and imago of the 

 great water-beetle (Dytiscus marginalis) ; the boat- 

 flies (Notonectidai) ; the various species of newts, 



