440 NOT 



Note 63, page 159. It is quite evident that, in point of fact, a modifi- 

 cation of the animal does not always result from a change from fresh to 

 salt water, and vice versa, since migratory fishes exhibit no effects from 

 the change of medium. Here it might, no doubt, be said that these 

 changes have not time to take effect, being too rapid. But there are 

 animals which occur simultaneously in fresh, brackish, and salt water, 

 and yet exhibit no differences, while other species display widely diver- 

 gent forms according to their habitat. To the former belong Crocodilus 

 biporcatus, Varuna literata, and others ; to the latter NerHina Mortoniana, 

 which in the sea is smooth, but which in brackish or fresh water often 

 developes spines, the distinguishing mark of the sub-genus Cliton, which 

 is characteristic of fresh-water streams. 



Note 64, page 159. The remark that only small animals occur in a 

 small area is an old one, but not altogether accurate. The saying is 

 familiar that the largest mammals occur only on continents. Even man 

 is to a certain extent subject to this law. Seafaring men, who pass the 

 greatest part of their lives, from their youth up, confined in an extremely 

 narrow space, are generally small, often below the middle height ; but 

 it may, at any rate, be questioned whether their small stature is a result 

 of this mode of life, or not rather of the nutrition, the lack of air, hard 

 labour, &c. In other cases, as those of land mollusca, insects, land- 

 vertebrata, and others, of which the same observations have been made, 

 it seems scarcely credible that their small size should be attributable to 

 the direct influence of a narrow area and to nothing else. Thus, for 

 instance, the fact that only small land-animals occur on small islands 

 of recent origin is easily explicable ; for as each of these has received 

 its fauna from beyond seas, the smallest animals have most easily reached 

 them, being the most easily transportable, while many large species 

 must be wholly excluded. All investigations on this question of the 

 influence of area ought at any rate to begin with fresh-water animals, 

 since in these the combined causes exhibit the least diversity. 



Note 65, page 163. The fact is not new. Mr. Jabez Hogg observed 

 it long ago, but he arrived at no general results from experiments, and 

 even his incidental observations are not particularly satisfactorily set 

 before us. (See Journal of the Microscopical Society, vol. ii. 1854, 

 ' Transact ions,' -p. 91.) Blanchard's observations are neither useful nor of 

 general interest. 



Note 66, page 166. In order not to occupy too much space in my 

 text, I have forborne from mentioning many details of my experiments ; 

 but a full report of them will be found in my treatise, Uebvr die Waclix- 

 thumsbedingungen der Lymnceus stagnalis (in Arb. aus dem Zool.-Zont. 

 Inst., Wiirzburg, 1874, vol. i.). So far as I can detect, every objection is 

 met by the facts there detailed ; some even are fully discussed, particu- 

 larly one not mentioned above that the relative proportion of the 

 surface of the water to its volume may affect the growth, because the 



