Page:Popular Science Monthly Volume 71.djvu/522

516 to the power of thought is a good word and will help forward toward good results.

In all that I have said, you will recognize that I have spoken constantly of the condition of the living material. If it is in the young state it has one set of capacities. If it is differentiated, it has, according to the nature of its differentiation, other kinds of capacities. We can follow the changing structure with the microscope. We can gain some knowledge of it by our present chemical methods. Fragmentary as that knowledge is, nevertheless, it suffices to show to us that the condition of the living material is essential and determines what the living material can do. I should like to insist for a moment upon this conception, because it is directly contrary to a conception of living material which has been widely prevalent in recent years, much defended and popularly presented on many different occasions. The other theory, the one to which I can not subscribe, may perhaps be most conveniently designated by the term—the theory of life units. It is held by the defenders of this faith that the living substance contains particles, very small in size, to which the vital properties are especially attached. They look at a cell and find that it has water, or water containing a small amount of salts in solution, filling up spaces between the threads of protoplasm. Water is not alive. They see in the protoplasm granules of one sort and another, in plants chlorophyll, in animals perhaps fat or some other material. That is not living substance, and so they go striking out from their conception of the living material in the cell one after another of these component parts until they get down to something very small, which they regard as the life unit. I do not believe these life units exist. It seems to me that all these dead parts, as this theory terms them, are parts of the living cell. They are factors which enable the functions of life to go on. Other conditions are also there, and to no one of them does the quality of life properly attach itself. Of life units there is an appalling array. The most respectable of them, in my opinion, are the life units which were hypothetically created by Charles Darwin in his theory of pangenesis. He assumed that there were small particles thrown off from different portions of the body circulating throughout the body, gathering sometimes in the germ cells. These particles he assumed to take up the qualities of the different parts of the body from which they emanated, and by gathering together in immense numbers in the germ cells they accomplished the hereditary transmission. We know now that this theory is not necessary, that it is not the correct theory. But at the time that Darwin promulgated it, it was a perfectly sound defensible theory, a theory which no one considering fairly the history of biological knowledge ought to criticize unfavorably. It was a fine mental achievement, but I should like also to add that of all the many theories of life units, this of Darwin's is the only one which seems to