EFFECTS OF VOLUME. 159 



doubt, contribute many important facts. But they would 

 certainly confirm the result obtained already : That there can 

 be no idea that a uniform change in one definite condition of 

 existence will produce a uniform effect on different animals. 63 

 This conclusion is self-evident when we reflect that the result 

 of any influence must be the resultant of a reciprocal action of 

 the external efficient force and of the inherent plasticity of 

 the organism which is influenced. 



III. Influence of the volume of water. It is well known 

 that the volume of water has a marked influence on the 

 growth of an animal, and on the size it finally attains. Every 

 lover of the 'gentle craft' of fishing for salmon, trout, or 

 other fi-esh-water fish knows that these fish are usually small 

 m small streams and lakes, and attain their full size only in 

 Hrge ones. This fact has often been proved in America as well 

 as Europe. All experimental zoologists know moreover that 

 it is often difficult, or even impossible, to rear fresh-water 

 animals in a small aquarium to the size which they grow 

 to under the normal conditions of a free life in rivers, ponds, 

 or even small pools. 64 This is attributed, if not without 

 exception, at least very generally, to a deficient food-supply. 

 Without any experimental enquiry, and under the tacit assump- 

 tion that all the other conditions such as the temperature, 

 the composition of the water, the amount of the oxygen it con- 

 tained, and the number of individuals were the same in the 

 aquarium as in small ponds or large lakes, it was asserted 

 that the smaller size of creatures in a small body of water was 

 due solely to the circumstance that the absolute quantity of 

 food at the disposal of each individual must necessarily be 

 smaller in a small volume of water than in a great one, and 

 hence be insufficient for the development of the animal's full 

 .size. Of course it cannot be disputed that a fish must remain 

 small if the food within its reach does not attain the daily 

 optimum. But it has never been investigated whether the 

 small size of the creatures in a small body of water is due, 

 without exception, to the small amount of food within reach, 

 either by proving that this actually was less than was indis- 

 pensable for the full growth of the animal, or by attempting to 



