8o EXPERIMENTAL EVOLUTION LECT. 



few days, and if the experiment is allowed to last 

 some months, we finally see that the inhabitant of the 

 largest volume of water is the largest in all ways, 

 that of the smallest being smallest, and that of the 

 intermediate aquarium being between the two as 

 concerns dimensions. Such is the general fact. But 

 many points are to be considered when we try to 

 explain it. The first explanation which suggests 

 itself is that in the larger space there is more to eat, 

 and that the pond-snails in small aquaria remain 

 small because they cannot secure food enough. This 

 objection and explanation are amply met by the fact 

 that in all my experiments care was taken to provide 

 superabundance of food in the form of aquatic plants, 

 and that the animals, whether in small or large 

 aquaria, had always at their disposal, a much larger 

 quantity of food than they could possibly eat, or than 

 they really did consume. So this explanation cannot 

 stand. Prof. Semper has thought of a curious 

 interpretation. He supposes that there exists in 

 common water some matter which, while not possessed 

 of nutritive properties, is conducive to growth and 

 development, and is a sort of incentive to both. If 

 the animal lives in a small body of water it has but 

 a small quantity of this matter at its disposal, and 

 does not grow as much as an animal in a larger 

 quantity of water. This interpretation is contradicted 



