82 EXPERIMENTAL EVOLUTION LECT. 



will and in different known degrees. In my first 

 series of experiments I used equal volumes of water, 

 but with different surfaces. One of the volumes, for 

 instance, was poured into a large-surfaced vase of 

 fifteen inches diameter, while the other was poured 

 into a vase of only four or six inches diameter. In 

 such cases I always found that the animals living in 

 the large-surfaced vase became much larger than the 

 others. Why so ? Is it that the water has better 

 aeration in the large-surfaced vase ? But this is of 

 no account at all. In the first place I would call 

 attention to the fact which I have repeatedly ob- 

 served since I began this series of experiments, 

 (and of which I am at present a daily witness), 

 that the aquatic plants which I used in my experiments 

 (Myriophyllum and Elodea canadense] do positively 

 thrive and grow much better in narrow-surfaced 

 vases than in large-surfaced vessels, in spherical 

 glass balloons with a long neck, in which the water has 

 but a very meagre surface contact with the atmosphere 

 (two centimetres diameter for instance), than in twenty 

 centimetres diameter vessels. This shows certainly 

 that in spherical vessels, with small surface, aeration 

 must be very good. On the other hand there is no 

 reason to think that aeration is better in one case 

 than in the other, as the water contains a large 

 amount of plants which ensure good aeration and 



