200 THE POPULAR SCIENCE MONTHLY. 



colonized from that source, and that the development of great 

 and cultivated nationalities was the result of ages of quiet resi- 

 dence in countries which favored by their climate and resources 

 the special phase of development which we here find recorded. 



As to the date of the planting of the first seeds of this civiliza- 

 tion we can only say that it is lost in the obscurity of the past. 

 Everything indicates that some of the monuments in the cate- 

 gory we have reviewed are among the oldest records of the human 

 race ; and it is certain that the gradual growth and spread of this 

 civilization, the long noonday of its maturity, and its progressive 

 decadence which began long before the advent of the Europeans 

 must be measured by thousands of years. Thus it will be seen 

 that in antiquity this indigenous and peculiar American civiliza- 

 tion takes rank with that of the Egyptians, Assyrians, Hindoos, 

 and Chinese, and in respect to culture, numerical importance, and 

 territorial area will bear comparison with either. 



--- 



WHAT ARE DIATOMS ? 



By EMILY L. GKEGOEY, 



OF BARNARD COLLEGE. 



SINCE the microscope has become so familiar in our homes and 

 ordinary places of resort, many terms are frequently heard 

 which have an unfamiliar sound. For example, a lady asked the 

 other day, with a laugh over the open confession of ignorance : 

 " What are diatoms ? I hear the word used very frequently, and 

 with such an air of acquaintanceship and familiarity, that one 

 must suppose they are the most common, every-day affairs, and 

 yet I must confess I have never seen one and don't know really 

 what they are." 



Thinking possibly there might be others interested in a brief 

 description of this curious plant, the following story is told of a 

 visit paid this summer to a gentleman said to know all about 

 diatoms. The plants in question are so small as to be seen only 

 with the aid of the microscope ; those of ordinary size, when mag- 

 nified about three hundred and fifty diameters, appear about a 

 quarter of an inch long. Others are much larger. They are curi- 

 ous little plants with a silica shell, which, in certain places, is pro- 

 vided with little apertures through which living parts of the 

 plant protrude. In this way they are enabled to move about 

 freely in the water by which they are generally surrounded, for, 

 though they are not all strictly water plants, they all need consid- 

 erable water to enable them to thrive, and so are always found in 

 wet places. 



