THE LIFE PROCESS IN VERTEBRATES. 265 



and animals. If a like study of typical plants were made, 

 it would reveal a corresponding gradation in form and 

 function from the lowest to the highest. It would lead 

 us to the conclusion that the plant life of the present ia 

 related to that of the past by direct descent. We should 

 find protoplasm the physical basis of plant life, and the 

 cell the unit of plant structure ; and at the bottom of 

 the series we should discover that there are free plant 

 cells which move about by pseudopodia and by cilia, 

 arid so combine in themselves the characters by which we 

 commonly distinguish animals from plants, that we are 

 almost unable to determine what characters predominate. 

 We thus come to the conclusion that life is a unit, and 

 that the higher and familiar forms are at extremes of the 

 divergent paths along which it has progressed. 



The study of animals should lead to a better under- 

 standing of man in his physical relationships. Not the 

 remotest object of the foregoing course is, that it shall 

 serve as an introduction to the elementary study of human 

 anatomy and physiology. The lowest animals have some 

 points of structure or of function in common with man ; 

 and these points become more numerous and more ap- 

 parent as we ascend the animal scale, until the greater 

 part of all that is written in the chapter concerning the 

 rabbit will apply almost equally well to the study of the 

 human body. The human body is governed by the same 

 laws, subject to many of the same necessities, and influ- 

 enced by many of the same instincts, as affect other 

 animal bodies ; and the student should know that it is on 

 the normal healthful activity of this animal body that all 

 the possibilities of happiness and usefulness in human life 

 depend. 



