AUTHOR'S PREFACE. 



IN the late evening of a varied and active life, I offer to tho 

 German public a work of which the undefined type has been 

 present to my mind for almost half a century. Often the 

 scheme has been relinquished as one which I could not hope 

 to realise, but ever after being thus abandoned, it has been 

 again, perhaps imprudently, resumed. In now presenting 

 its fulfilment to my contemporaries, with that hesitation which 

 a just diffidence of my own powers could not fail to inspire, 

 I would willingly forget that writings long expected are 

 usually least favourably received. 



While the outward circumstances of my life, and an irre- 

 sistible impulse to the acquisition of different kinds of know- 

 ledge, led me to occupy myself for many years, apparently 

 exclusively, with sep arate branches of science, descriptive 

 botany, geology, chemistry, geographical determinations, 

 and terrestrial ma gnetism, tending to render useful the ex- 



