24 INTRODUCTION. 



structure which it was difficult to investigate. And now 

 by referring the result from one kingdom to the other, it is 

 to be hoped that much more rapid progress will be obtained 

 than before."* Many problems connected with nutrition 

 and reproduction in animals will probably be solved by a 

 more careful observation of these functions in vegetables. 

 A knowledge of cell-life, now universally admitted to be 

 the basis of all scientific physiology, can be best acquired 

 by examining the cells of plants which are much larger 

 than those of animals, and visible to ordinary microscopes 

 at every epoch of their development. The differences which 

 exist among the organic productions are not so great as is 

 commonly thought. There is a oneness in nature which 

 has yet to be understood and appreciated. 



One of the most striking differences of organization be- 

 tween the higher orders of animals and plants consists in 

 the presence of a nervo-muscular system in the former of 

 which the latter are totally deprived. This nervo-muscular 

 system, which is essential to animality, appears to be gradu- 

 ally developed in vegetable life, which thus becomes in- 

 separably bound up with the exercise of the animal func- 

 tions. It is gradually developed in the inferior orders of 

 animated being, and is manifested most perfectly in man, 

 and those animals the most closely allied to him in organi- 

 zation. 



Now comparative anatomy shows that the animal func- 

 tions of sensation and voluntary motion, manifest themselves 

 in proportion to the more or less perfect condition of the 

 organs appropriated to their exercise. In man, the highest 

 vertebral animal, the organs of the senses and the muscular 



* Lectures on Comparative Embryology, delivered before the Lowell 

 Institute in Boston, by Louis Agassiz. 



