THE NATUEE OF THE PROBLEM. 3 



composed. This anatomical knowledge is of extreme 

 importance ; and it is supplemented by the embryo- 

 logical discovery that each of the higher multicellular 

 organisms is developed out of one simple cell, the 

 impregnated ovum. The " Cellular theory," which 

 has been founded on that discovery, has given us the 

 first true interpretation of the physical, chemical, and 

 even the psychological, processes of life — those myste- 

 rious phenomena for whose explanation it had been 

 customary to postulate a supernatural "vital force" 

 or "immortal soul." Moreover, the true character of 

 disease has been made clear and intelligible to the 

 physician for the first time by the cognate science of 

 Cellular Pathology. 



The discoveries of the nineteenth century in the 

 inorganic world are no less important. Physics has 

 made astounding progress in every section of its 

 province — in optics and acoustics, in magnetism and 

 electricity, in mechanics and thermo-dynamics ; and, 

 what is still more important, it has proved the unity 

 of the forces of the entire universe. The mechanical 

 theory of heat has shown how intimately they are 

 connected, and how each can, in certain conditions, 

 transform itself directly into another. Spectrum 

 analysis has taught us that the same matter which 

 enters into the composition of all bodies on earth, 

 including its living inhabitants, builds up the rest of 

 the planets, the sun, and the most distant stars. 

 Astro-physics has considerably enlarged our cosmic 

 perspective in revealing to us, in the immeasurable 

 depths of space, millions of circling spheres, larger 

 than our earth, and, like it, in endless transformation, 

 in an eternal rhythm of life and death. Chemistry has 

 introduced us to a multitude of new substances, all of 



