192 REPTILES AND FISH 



ing on to struggling prey, the latter should be weakened, 

 even only a little, by injection of slightly poisonous saliva, 

 for the prey would then be more easily swallowed. Having 

 started thus one can see how the poisonous quality could 

 be developed. For the more quickly the struggling 

 animal succumbed, the less time would the snake need 

 to hold on to it, and the less chance would there be of 

 its escaping, until, with the increasing strength of the 

 venom, a stage would be reached when the snake would 

 only need to strike once, and quietly wait a few minutes 

 until the prey, at once seriously poisoned, falls helpless 

 and dies. 



An extremely interesting observation is here quoted 

 from a letter in Nature by Professor Poulton, dated 

 January 2, 1918. Professor Poulton quotes the observation 

 for another purpose, but it is so exactly illustrative of my 

 point that it is here repeated. Dr. G. A. K. Marshall 

 says : " When happening to look over a low stone wall 

 near Estcourt, Natal, in 1897, I chanced to observe a 

 small snake in the very act of striking a frog. After being 

 bitten the latter hopped of! at a great pace, and I was 

 rather siirprised to see that the snake made no attempt 

 at pursuit, but merely followed in a very leisurely manner " 

 (the italics are mine). " Seeing that the frog had come to 

 a standstill at a considerable distance ofif, I crept along 

 under the wall so as not to disturb the snake, and on 

 getting near the frog I looked cautiously over the wall 

 to see the end of the tragedy. The snake was still 

 some way behind, approaching steadily, and on reaching 

 its victim stood watching it for some moments with 

 its head raised, the frog meanwhile sitting trembling 

 in front of it. At last the snake seized its prey and suc- 

 ceeded in swalowing it after hut feeble resistance. It 

 seemed clear that the trembling and inability to escape 

 on the part of the frog were simply due to the action of 

 the poison injected at the first bite." 



