6oo THE POPULAR SCIENCE MONTHLY. 



which is itself the result of past environments, and to set it up as a 

 barrier against the influence of new environments. 



Descending still deeper, scientific mQn have sought to explain the 

 constitution of living things, their production, and the existence of 

 the groups into which we find them divided. Hence three theories 

 wljich have had different fortunes the cellular theory, the doctrine 

 of spontaneous generation, and transformism. 



Schwann, applying to the animal organism Schleiden's discoveries 

 in ve<^etal organisms, showed that the tissues are formed of primor- 

 dial, i. e., irreducible, elements, called cells, though often these ele- 

 ments have no cavity and are simply rounded masses. The egg, 

 which is the starting-point of all animal organisms, is at first merely 

 a cell, and develops by producing within itself other cells, which are 

 the primitive materials of the living being. All that the organism is 

 comes ultimately from the cells, which are converted into living tis- 

 sues. They adhere to one another end to end, and become flattened, 

 or lengthened, or ramified ; or they unite and form one common cavity, 

 keeping their walls only at points where they are not in contact, thus 

 forming tubes, or fibres, as, for example, in the histological elements 

 of muscles and nerves. 



Some authors have explained the production of cells on the hy- 

 pothesis of a true spontaneous generation. According to them, cells 

 are organized in a saline solution, the first step being the deposit of a 

 nucleolus, around which there forms an envelope called the nucleus, 

 and finally, at a greater distance, a second envelope, or cell-wall. 

 But no actual experiment has ever been made on the production of 

 cells in this way, and hitherto we have no knowledge of a cell being 

 produced save from a cell. Of this famous theory so much yet re- 

 mains, viz., that the cell, whatsoever its form and whatever modifica- 

 tions it may have received, is ever the basis of the vital phenomena. 



"One only elementary form " (says Virchow) "runs through the whole or- 

 ganic world, remaining ever the same ; in vain would we attempt to substitute 

 any thing else for it ; there is nothing that can take its place. We have come 

 to regard even the highest formations, whether plant or animal, as being the 

 sum of a larger or smaller number of like or unlike cells. The tree represents a 

 mass put together according to a certain law ; each of its parts, leaf or root, 

 trunk or flower, contains cellular elements. The same is true of the animal 

 world. Eacli animal represents a sum of vital units, every one of which has in 

 itself the perfect characters of life. . . . The higher organism, the individual, is 

 always the result of a sort of social organization, of the union of sundry elements 

 combined ; it is a mass of individual existences, dependent on each other, though 

 their dependence is such that each element has its own proper activity; so that, 

 whatever impulse or excitation other parts may give to the element, the result- 

 ing function nevertheless emanates from the element itself, and is its own." 



The question as to how living bodies arc produced gave rise, a few 

 years ago, to discussions which have again brought to the surface a 



