If all pedunculated cirripedes had become extinct, and they have suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiae in this latter family had originally existed as organs for preventing the ova from being washed out of the sack? There is another possible mode of transition, namely, through the acceleration or retardation of the period of reproduction.

This has lately been insisted on by Professor Cope and others in the United States.

It is now known that some animals are capable of reproduction at a very early age, before they have acquired their perfect characters; and if this power became thoroughly well developed in a species, it seems probable that the adult stage of development would sooner or later be lost; and in this case, especially if the larva differed much from the mature form, the character of the species would be greatly changed and degraded.

Again, not a few animals, after arriving at maturity, go on changing in character during nearly their whole lives.

With mammals, for instance, the form of the skull is often much altered with age, of which Dr.