264 The Unity of the- Organism 



they are so like ordinary living things in their general' modes 

 of life despite their excessive minuteness, and, as judged by 

 ordinary anatomical standards, despite their structural 

 simplicity. Like other organisms they feed and respire and 

 propagate their kind ; and they survive in an active condition 

 only within a range of temperatures not greatly different 

 from that which conditions the lives of other beings. But 

 striking as is their similarity to organisms generally in 

 these attributes-in-common, still more striking is their 

 specific diversity among themselves. That organisms so 

 small as to be barely visible, or even invisible to the highest 

 powers of the microscope, should still be subject to the gen- 

 eral principles of specific differentiation in habits of life 

 and hereditary transmission as are the largest, most com- 

 plex organisms, seems to me one of the marvels of the living 

 world. 



In this field as in so many others, the facts which touch 

 human welfare the most vitally are best known. Take the 

 case of smallpox and chickenpox. These are both diseases 

 of the human skin and are so much alike as to be often con- 

 fused even by physicians not especially experienced in this 

 field ; yet to the expert the difference between them is de- 

 clared to be positive and unmistakable. "Smallpox has 

 rather severe prodromes, backache, head-ache, fever, and 

 sorethroat, the rash appearing on the third and fourth day. 

 Cliickenpox usually has light or no prodromes, the rash ap- 

 pearing on the same day or within twenty-four hours as a 

 rule. In both diseases the face, chest, back, arms, hands, 

 legs, and feet are likely to show eruptions, but chickenpox 

 tends to show the greatest number of spots 'under cover,' 

 i.e., on the parts usually covered by clothing, while smallpox 

 tends to show the majority on the face, neck, arms, wrists, 

 and hands, rather than on the body. The skin lesions them- 

 selves differ very markedly, the typical lesions of chickenpox 

 being superficial, thin-walled, high, rounded, and filled with 



