Scientific Lectures. 23 



having silicious loricoe or shells ; and of these, the hrachionus urceo- 

 laris, so called from its elegant urn-shaped lorica, is among those 

 most frequently met with. Of this animal more than one million 

 individuals could be easily packed in the space of a cubic inch. But 

 this is one of the forms which may perhaps be properly called gigan- 

 tic. It is even large enough to be discerned by the naked eye ; not 

 indeed in its shape and structure, or the puzzling and paradoxical 

 appearances presented in the movements of its marvelously delicate 

 organs, but as an animated point rapidly speeding its way through 

 the watery drop which forms its ocean. Of this and allied genera or 

 families, there are, however, others which, though equally com- 

 plex in their organizations, are vastly more minute. Of the genera 

 Salpina, Euchlanis^ Monostyla, and others, all having elegantly 

 sculptured silicious shells, there are some of which at least 10,000,000 

 to 20,000,000 could find room in a cubic inch of space. 



Of simpler forms of life, the minuteness is still more wonderful. 

 The monadina^ monads, are little spheroidal sacks having a single 

 thread-like filament proceeding from the mouth, which seems to serve 

 the double purpose of securing food and aiding locomotion. Difierent 

 species vary in their dimensions, few exceeding the 1,000th of an 

 inch in diameter, and some being not more than one 12,000th. The 

 monads are commonly regarded as being true animals, although some 

 naturalists, among whom may be mentioned our own Agassiz, have 

 held that they are but the germs of various kinds of algae. They 

 are distinguished by great activity of movement, and if their move- 

 ments are not directed by a manifest exercise of will, it is all but 

 impossible for the observer to escape from the illusion that they are so. 



In order that some idea may be formed of the exceeding minute- 

 ness of these objects, I will no longer suppose a space so enormous as 

 a cubic inch to be filled with them. I will suppose a cube of only 

 one-tenth of an inch on the edge. A little block of oak of such 

 dimensions would weigh about a quarter of a grain. It might be 

 rej)resented in bulk by a drop of water, such as I might lift on the 

 point of my pencil. Yet within this insignificant space may be easily 

 contained, of some of the smaller forms of these oi'ganisms, a num- 

 ber not less than two thousand millions ; that is to say, more than 

 double the number of the human inhabitants of the entire earth. 

 Even this will fail to convey an adequate idea of the extreme minute- 

 ness of the objects we are considering ; for such numbers as millions 

 and thousands of millions are totally inconceivable by the mind. The 



