how, under appropriate conditions, a cell may, in the course of untold years, from the same simple beginning, give origin to the human race." Biology teaches us that apparently insignificant forces may work large increments of change, when the time of their operation is unlimited. It teaches us that there is a simplicity in nature, greater than the untutored mind could suggest; that the agents are so apparently simple and in- significant that the common sense-or, as remarked, the common ignorance"-of mankind passes by them without thinking them worthy of attention. But, says Cole- ridge, to whom I have already referred, "the positions of science must be tried in the jeweller's scales, not like the mixed commodities of the market on the weigh- bridge of common opinion and vulgar usage.'
Every advancement of science exhibits to us new illustrations of great results from causes apparently the most inefficient. Tyn- dall has lately shown the probability that the green parts of plants may be formed by the decomposition of carbonic acid, through
