RELATION OP PLANT PHYSIOLOGY TO OTHER SCIENCES. 443 



seems to be one great totality whose parts are in reciprocal relation and 

 mutual interaction, like the organs of a living organism. I would like to 

 consider the unity of science under the figure of a tree of life which 

 grows upward from the earth from which one part takes its power and 

 nutriment and in which it finds its support. The parts of this tree — 

 roots, stem, branches^jind whatever they may all be called — appear to 

 us externally different, but within they belong together; they stand 

 among each other continually in helpful interaction. As the organs, 

 so are the tissues adjusted to each other, and not one of the millions of 

 cells in a tree is without purpose, and if each cell does not stand in 

 fast relation to all others, how also need not each single scientific ques- 

 tion be related to all others'? This can as little destroy the unity of 

 science as the unity of organic structure of a tree can be destroyed 

 by the fact that each cell does not stand in mutual relation with every 

 other cell. 



Wonderingly we see this tree of science develop and broaden out; 

 but for this provision is made, namely, that this tree shall not grow 

 even into the heavens. 



After thousands of years of seeking and groping, mankind has finally 

 discovered how he may reach high aims of knowledge in spite of the 

 limitations of his mind, by the often slow and heavily progressing induc- 

 tive method, and the principle of the division of labor, which first leads 

 to division, but after a rich harvest binds all together. It becomes 

 even clearer that the synthetical mental work, flowing out of the prin- 

 ciple of division of labor, must lead to even greater conceptions, and 

 that the number of men must be even greater who, raising themselves 

 above the level of specialists, will be investigators in the best sense of 

 the word. 



Held in bounds by the exact nature of its work, science strides for- 

 ward, ever attaining more and more of what is seemingly unattainable 

 to the human mind, and likewise ever more clearly recognizing the 

 unattainable as unattainable. Indeed, more and more we come face to 

 face with the limits of our knowledge. To the Grecian thinkers it 

 seemed a play that allowed the living to spring out of the lifeless, plants 

 or animals from slime or damp earth. But the inductive method has 

 led us thus far to know that, so far as observation can go, the living 

 can arise only from the living. Even the smallest known living beings, 

 the bacteria, do not come into being parentless, as not long since the 

 last notes of retreat of the defense of spontaneous generation declared. 

 In the organism itself, all that is living proceeds only out of the living — 

 the cell from a cell, the nucleus from a nucleus — and the smallest plastid 

 lying on the very border of microscopic observation proceeds from its 

 like. The possibility enlarged upon by many naturalists, that in the 

 organism living constituents can arise spontaneously, is only a reaction 

 of the old doctrine of spontaneous generation; for, so far as investiga- 

 tion shows, there can rise within the organism organized substance 



