common to North America and Europe. 475 



cies and individuals to be more able to pass than others, and 

 that many become extinct, from inability to accomplish it. 

 Under this point of view, a hiatus, rather than a regular pas- 

 sage, is required between a species and that whence it is sup- 

 posed to be derived, just as two crystals may occur, nearly 

 identical in composition, but without an insensible gradation 

 of intermediate forms,* the laws, both of organic and inor- 

 ganic matter, requiring something definite ; whence the rarity 

 of hybrids and monsters, themselves subject to established 

 laws. 



My meaning will, perhaps, be better understood by one or 

 two illustrations. We all know that marine and fluviatile 

 mollusca have their peculiar distinctive characters. Let us, 

 then, suppose a species of iMelania to inhabit a stream into 

 which a salt lake effects a discharge, the saline mixture 

 being, at first, so much diluted as not to occasion the animal 

 much inconvenience, until by the gradual enlargement of the 

 outlet of the lake, the amount of the foreign ingredient is so 

 much increased, that the mollusk finds great difficulty in liv- 

 ing, and must eventually perish, unless it can accommodate 

 itself to the saline medium. The form, however, not being 

 marine, the extreme case is presented, of a necessity to change 

 into a different genus — Fusus, for example. Now there is a 

 shell confined to the Holston River and its branches, which 

 is, to all appearance, a Fusus, so that Say, the leading Ameri- 

 can conchologist of his day, called it Fusus fluvialis. Accord- 

 ing to the lamarckian hypothesis, this mollusk may have de- 

 scended from individuals of the more widely-distributed and 

 nearly-allied Melania armigera, Say; and its transmutation 

 (though now a fluviatile species) may have been accomplished 

 by the agency of salt water. Now, although we will not 

 assert that the salt water they inhabit is the cause of the 

 si|)honal canal in a large proportion of marine univalves, or 

 even insist that the want of this medium has some connection 

 with its absence in the numerous freshwater species, we are 



* The same mineral may crystallize with three, six, or twelve angles, but not with 

 five or seven. Are the phases of organic morphism subject to less definite laws ? 



