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.Murie has given some striking instances with seals.

Every one knows how the horns of stags become more and more branched, and the plumes of some birds become more finely developed, as they grow older.
Professor Cope states that the teeth of certain lizards change much in shape with advancing years.