The Transformation of the Gills into Lungs in Frogs.
Is it true that certain conditions in nature as well as its characteristics have led to the transformation of a fish that breathes with its gills into a frog that breathes with its lungs? If such is the case, how does one explain all this?†
The illustration on the left depicts the phase-by-phase development of a frogís eggs into an adult frog. While frogs have throughout their lifetime and for generations been known to live in very shallow water to such depth that enables the upper half of their bodies to stay above the surface as soon as they turn adult, during pre-adulthood, however, when their bodies are so small that they are completely submerged underwater, they breathe in air with their gills.
During adulthood, with the upper half of their bodies above the water surface, they have to adapt themselves to their natural surroundings and thereby start to breathe with their lungs.
As can be seen in the illustration, the hatched egg turns into a fish breathing in air with its gills. Not long after, the fore and the hind legs appear, while the tail disappears bit by bit, thus turning the creature into a frog that lives on land and breathes with its lungs. The blue line represents the surface of a swamp. Thus, while on the one hand metamorphosis in creatures occurs out of their desire to survive, on the other hand it represents a process triggered by the unusual external condition, whereby it has to undergo some bodily adaptation in order to ensure its survival.
The metamorphosis in a frog is sparked off by the frogís turning adult, when its body grows to such a size that it simply canít help preventing its upper part from emerging above the water surface, thereby forcing it to adapt its body (which earlier had been adapted to underwater conditions)† to† above-water-surface conditions.
Now that such a condition has been going on for generations, for more than millions of years, the effects it has on the growth of frogís egg into an adult frog must be significant. Although in its mid-evolution phase it has been able to move about, e.g. from a shallow place to a deeper place, yet it still inherits the metamorphosis that its forebears has long undergone. It is all these that have caused the life-cycle of a frog or other creatures that metamorphose to remain so until today.