THE BEGINNINGS OF LIFE. 285 



research., The surface of the earth is clothed with 

 living things of all kinds, animal and vegetal, which 

 are not only continually throwing off organic particles 

 and fragments during their life x , but are constantly 

 undergoing processes of decay and molecular disinte- 

 gration after their death. The actual reproductive 

 elements of these living things are extremely small 

 in bulk when compared with other parts which are 

 not reproductive. When, moreover, it is considered 

 that in the neighbourhood of populous cities (the air 

 of which alone exhibits this very large quantity of 

 impalpable, mixed with palpable, organic dust), there 

 is constantly going on a wear and tear of the textile 

 fabrics and of the organic products of various kinds 

 subservient to the wants of man ; and that the chim- 

 neys of manufactories and dwelling-houses are con- 

 tinually emitting clouds of imperfectly consumed organic 

 particles, some idea may be gained of the manifold 

 sources whence the organic particles and fragments 

 found in the atmosphere may emanate, and also as to 

 what proportion of them is likely to be composed of 

 living or dead reproductive elements. 



1 Epithelial cells and the debris of such bodies are generally obtainable 

 from the air of ill-ventilated dwelling-rooms, when it is passed through 

 the aeroscope. These off-cast units, as well as pus corpuscles, become 

 much more abundant in hospital wards especially when they are over- 

 crowded and contain patients with open wounds. The presence of such 

 off-cast elements and particles in the atmosphere is one important 

 means by which the spread of contagious diseases amongst men, and 

 also amongst the lower animals, is brought about. (jSee Appendix E, 

 p. cxliv.) 



