BIOLOGY AND HUMAN WELFARE 5 



minimum waste of time and money. These laws of heredity- 

 were worked out not on sheep but on other animals, especially 

 flies, of which many generations each with very numerous 

 individuals may be obtained within a single year. But the 

 broader generalizations discovered through the breeding of 

 flies in the laboratory were found to be applicable to sheep on 

 the farm, and by approaching the subject in this way enor- 

 mous amounts of money were saved and, besides, many things 

 were learned which could never have been learned from sheep. 

 Similarly the generalizations were apphed to the breeding of 

 chickens to produce varieties which would yield the greatest 

 number of eggs; to cattle to produce the largest yield of milk 

 and butter; to wheat to produce the best flour; to silkworms 

 to produce the best silk; and to other animals and plants to 

 increase their value to us. 



Living in large herds sheep and cattle are especially subject 

 to parasites and diseases which must be carefully studied in 

 order that their ravages may be prevented. Of these diseases 

 we shall mention only one, at the same time cautioning the 

 reader to remember that there are many others, some much 

 less well known. The "liver rot" of sheep is widely spread 

 and often disastrous, killing, it is said, not less than a million 

 sheep a year in the United Kingdom alone. This disease is 

 the evidence of the activity of a flat-worm or fluke, only one 

 of several hundred kinds of these creatures, which lives in the 

 sheep's Hver. Each fluke produces half a mfllion or more eggs 

 which pass out of the sheep and fall to the ground. The rain 

 washes them into pools and ponds where they hatch, giving 

 forth an active conical creature, exceedingly small, which swims 

 about until it finds a snail into which it bores its way. Fafling 

 the discovery of a snail it perishes; but unfortunately nature 

 provides plenty of snails for it. Within the snail it grows into 

 a sort of sac which in its interior develops another t>'pe of 

 young; within these last more young develop, some like the 

 parent, and some hke minute poHyu'ogs which emerge from 

 the snail and swim away, climb up a grass blade, lose their 



