30 INSECT PESTS 



even the most seemingly negligible creatures appear to 

 be invested with the " yes " or " no " of some higher 

 creature's continuance on the earth. Take the present 

 instance. A sheep is grazing on the hillside on a hot day, 

 and feehng thirsty comes down to the stream for a drink. 

 In less than a year that sheep is dead, having become an 

 emaciated shadow of what it was ! But how can the 

 simple act of 'going down to the brook be answerable 

 for such a loss ? We must investigate without delay. 

 First we shall test the water, and find it quite pure. 

 Then we shall have received the veterinary siu-geon's report 

 of Fluke or Liver Rot, obtained from the stream. We 

 return and take out a few bucketful s of stream bottom and 

 examine it. Here are some tiny snails, no bigger than 

 a glass bead. Let us dissect one and put it under the 

 microscope. What's this ? The Uver of the snail contains 

 some half-dozen or so tiny oval creatures like that shown 

 in the sketch, and which is exactly like the larger drawing 

 of the liver fluke ! It is simply a case of " like parent, 

 like child." And yet sheep don't eat water-snails ? No, 

 and we have here a strange hfe cycle presented to us, where 

 two totally different creatures, a sheep on the hillside and 

 a httle water-snail in the brook down below are closely 

 connected with one another by a third creature, a parasite 

 that requires quite an elaborate chain of circumstances 

 to be fulfilled ere it can bring about its complete round of 

 development. The Liverfluke begins its existence as a 

 minute egg, embedded in the droppings of the sheep. 

 It has been found that one female liverflul^e can produce 

 over 40,000 eggs, so that there is plenty of margin for 

 loss. Nature always provides abundantlj'^ for these cases 

 where there are many chances against the young creature 

 ever reaching maturity ; as is also seen in, say, the cod- 

 fish, wtose countless thousands of ova are decimated and 

 re-decimated again and again by enemies at every stage 

 of their career from their entry into the world as spawn 

 to the full-sized candidate for Billingsgate market. It is, 



