458 NOTES. 



the Limacidae. From mere external resemblance a host of shells from 

 India, Persia, &c., have constantly been described as belonging to 

 Helicarion which, so far as it has hitherto been possible to inves- 

 tigate the creatures anatomically, all belong to the typical genera of 

 the neighbouring Indian mainland, to which indeed they often ex- 

 hibit but little similarity even in their shells. The Philippine mollusc 

 Pfeiffcria micans is often placed under Yitrina, and the shell certainly 

 has some likeness to that of Vitrina, but the animal is in every par- 

 ticular a true Cochlostyla (and thus a true Helix), and is one of the 

 innumerable variations of this variable genus, the structure of the shell 

 completely disguising its true character. If these and the other 200 or 

 so of species of Cochlostyla could be discovered somewhere in a fossil 

 state, geologists would undoubtedly make at least eight distinct 

 genera of them. This instance must here suffice to justify the assertion 

 I have made. 



Nate 110, page 289. Wagner's phrase, which I have somewhat altered 

 in the text, runs as follows : ' Each closed cycle of forms (a species or 

 constant variety) originates in a mechanical process of isolation and 

 colony-formation by individual emigrants from a parent-stock capable 

 of variation ; the indispensable conditions of the formation of such a 

 cycle are variability and inheritance. The sum of morphological cha- 

 racters which distinguish it are the result from the sum of differences in 

 the external conditions of life on the one part such as food, climate, 

 character of the soil as supplied by the habitat of the isolated colony, 

 when compared with the native province of the old stock, and from the 

 sum of phyletic and individual capacity for variation on the other part 

 as imported by the colonist itself, and transmitted as morphological 

 characters to its progeny, and posterity by direct descent. The constancy 

 of the new form always depends on a lopg-continued period of iso- 

 lation.' 



Note \\\,page 291. I cannot understand in any other sense the vari- 

 ous passages in which Wagner distinctly opposes his theory of ' isolation ' 

 to the ' struggle for existence.' I will here quote only one passage : 

 ' The AchatinellcB are harmless vegetable-feeders, content with any 

 situation, and their overwhelming multiplication is kept within bounds 

 not by the pursuit of enemies, but by epidemics. They have no vital 

 struggle to carry on for food, since this is supplied in any quantity by 

 the abundant herbage of the soil, nor can we discover that any struggle 

 for propagation can take place among them, since each animal is herm- 

 aphrodite, and pairs with any other. If here and there one of these snails, 

 which generally find sufficient shelter by a rapid retreat into their shell, 

 is by chance devoured by a bird or a predatory beetle, or accidentally 

 crushed by a grazing beast, these are purely accidental occurrences, which 

 would be far less likely to reduce their numbers than the constant per- 

 secution to which the ladybird, for instance, is exposed. Nature has at 



