28 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



might possibly be the best, although even then, it is per- 

 haps doubtful; but in physiology there can be no doubt 

 that the latter is best. It is so for two reasons: first, 

 because we take hold at once of the interest of the 

 student, and, second, because functions are fully sepa- 

 rated and declared and therefore intelligible only in the 

 higher animals. As we go down the scale functions are 

 more and more merged into one another, and therefore 

 more and more indistinct, until in the lowest animals 

 each part performs all the functions, but in so imperfect 

 a way that it is impossible to understand them unless 

 we have already studied them in their separated and 

 perfect form in the higher animals. " In higher animals 

 the functions rise to the surface; in lower animals they 

 are deeply buried. We grope in vain unless we find the 

 key by the study of the higher animals." * 



Our plan, then, will be to take up each system of 

 organs first in man; then, running down the scale of 

 vertebrates, show the modifications and simplifications 

 which we find there. Then we shall take the other de- 

 partments of the animal kingdom and treat them in the 

 same way, but much more cursorily. 



* Foster, Nature, vol. lvi, p. 437, 1897. 



