Note 75 — Chapter V · EVOLUTION.

It has recently been shown that certain Crustacea vary not only from species to species, but from genus to genus, when living in water of different degrees of saltness. By continued dilution of the salt water an Artemia was developed into another species, and this again into a Branchipus—a genus of large dimensions, with an extra abdominal segment, and a different tail; a genus, moreover, which is propagated sexually, whereas the Artemia is parthenogenetic, as a rule. See Nature, 1876, June 8, p. 133.

The exceeding importance of this fact is, that it proves specific and even generic differences to originate simply through the gradual changes of the medium and the adaptation of the organism to these new conditions. It also disproves the very common notion—adopted even by Mr. Darwin himself—that “organic beings must be exposed during several generations to new conditions to cause any appreciable amount of variation.” Again, “Natural Selection, if it be a true principle, will banish the belief of any great and sudden modification of structure.”—Comp. note to § 121, p. 132.