Again, with respect to the singular fact that many terrestrial and fresh-water animals do not undergo any metamorphosis, while marine members of the same groups pass through various transformations, Fritz Muller has suggested that the process of slowly modifying and adapting an animal to live on the land or in fresh water, instead of in the sea, would be greatly simplified by its not passing through any larval stage; for it is not probable that places well adapted for both the larval and mature stages, under such new and greatly changed habits of life, would commonly be found unoccupied or ill-occupied by other organisms.
Gainera, lehor eta ur gezatako animalia askok metamorfosirik ez izatearen egitate bitxi horri dagokionez-talde berberetako itsas kideek, ostera, eraldaketa bat baino gehiago jasaten duten bitartean-, Fritz Müllerrek hauxe adierazi du: animalia batek itsasoan bizi beharrean lehorrean edo ur gezatan bizitzeko burutu behar duen astiroko eraldaketa eta egokitzapen prozesua asko erraztuko litzatekeela, inolako larba-egoerarik iragan beharrik ez duelako; gertatu ere, nekez gertatuko baita beste organismo batzuek bete gabe edo erdizka beterik topatzea, bai larbaren eta bai helduaren egoerarako (halako bizi-aztura berri eta zeharo ezberdinetarako) aproposak diren lekuak.