THE ORIGIN OF NERVES. 273 



obtain of the powers and contrivance of the forces of life, 

 as displayed in the fashioning of a complicated body from 

 apparently the very simplest of materials. Some such 

 thought, doubtless, stirred the great Harvey, one of the 

 first to study the development of animals, when he main- 

 tained in his " Exercitations " that "in the generation of the 

 chicken out of the egg, all things are set up and formed 

 with a most singular providence, divine wisdom, and an 

 admirable and incomprehensible artifice." 



The importance of the study of development has, how- 

 ever, been greatly increased of late years, through the grow- 

 ing force of the idea that in the development of animals 

 and plants we may obtain a clue to their origin and manner 

 of descent. Starting with the idea supported by well-nigh 

 every consideration which natural science can offer that 

 the living beings around us have been evolved from pre- 

 existing forms of life, it is held that in their development 

 we may see illustrated the various stages through which their 

 ancestors have passed, and through which their modern and 

 existing forms and structures have been produced. The 

 development of a living being is thus regarded as teaching 

 us how living nature has been evolved; the "why" is a 

 subject upon which the fullest research sheds no light, and 

 regarding which even the boldness of speculation has as yet 

 pronounced no opinion. To use the words of Mr. Darwin 

 himself, "Community in embryonic structure reveals com- 

 munity of descent ; " and again, " Embryology rises greatly 

 in interest, when we look at the embryo as a picture, more 

 or less obscured, of the progenitor, either in its adult or 

 larval state, of all the members of the same great class." 



Applying the principle that in development we find a 

 clue to the origin of the structures and organs of living 

 beings, we purpose to investigate briefly the history and 

 origin of that part of the animal frame which is concerned 

 with the maintenance of relations between the organism and 

 the outer world, the nervous system. We may endeavour, 

 in other words, to apply the foregoing principle to explain 



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