4 PLANT-ANIMALS [CH. 



animals has laid the foundation of a great and 

 increasing body of knowledge with respect to the 

 cause of malaria, sleeping sickness, and other tropical 

 diseases. 



In the second place, it may be urged that, the 

 more complex the organism, the more difficult it is to 

 use the results of observations upon it for the purpose 

 of generalising on important biological problems such 

 as those of the origin of instinct and habit, or of the 

 meaning of heredity and the course of evolution. 

 The higher the organism, the more it has covered up 

 the tracks along which the species to which it belongs 

 has travelled. For this reason alone, the study of the 

 lower organisms is not only to be justified but also 

 urged on zoologists as one bound to lead to results 

 of the greatest value. 



In the third place, it has yet to be proved that 

 the higher animals differ in any fundamental respect 

 from more lowly forms of life. Hence, if, as a 

 physiologist must hold, such differences as exist 

 between higher and lower forms are differences of 

 degree and not of kind, it follows that an increased 

 knowledge of the nature of the lower organisms 

 connotes also an increase in knowledge with respect 

 to the higher organisms. 



On these grounds, the patient and exhaustive 

 study of the lower organisms is to be justified. 

 Nay more, if the reasons for this study are valid they 



