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It is evident, therefore, that the Mexican Axolotls, although they have been perennibranchiate for a great number of generations, have not lost the hereditary tendency to the metamorphosis which changes the larvae of Amblystoma elsewhere into an air-breathing terrestrial animal.
But there is no reason to suppose that a congenital defect of thyroid arising as a mutation was the original cause of the neoteny, i.e. the peisistence of the larval or aquatic, branchiate condition. Such a supposition would imply that the association between Axolotls and the peculiar Mexican lakes, supplied with oxygenated water by springs at the bottom, was purely accidental.
There seems no reason to doubt that modern fish, as a whole, quite equal in size the piscine fauna of any previous geological age. It is somewhat different with the next great vertebrate group, the amphibians, represented in our own world only by the frogs, the toads, the newts, and the axolotls.
In the little tank below, where the water lies so clear that everything is visible upon its bottom, one may see axolotls creeping. They are water-salamanders, but they have a strange history. Like frogs, they pass through a series of changes, and the larval is very different from the adult form. Our last evening at Huixquilucan, I went out to purchase native garments.
We have here, I think, an example of the essential difference between mutations and somatic modifications. Absence of the gametic factor or factors for pigmentation results in albinism, and no amount of exposure to light produces pigmentation in albinos, e.g. albino Axolotls which are well known in captivity. Absence of light, on the other hand, prevents the development of pigment.
The most important of these conditions seems to be abundance of oxygen in solution in the water, and the next in importance abundance of food in the water. Recently it has been shown that the metamorphosis may be induced by feeding Axolotls on thyroid gland.
Yet the Redfins, as typified by Guinevere, have done both, and given time enough, they may emulate or surpass the achievements of larval axolotls, or the astounding egg-producing maggots of certain gnats, thus realizing all the possibilities of froghood while yet cribbed within the lowly casing of a pollywog.
The fact, however, that Axolotls require special treatment to induce metamorphosis seems to show that they have distinctly less congenital tendency to metamorphosis than larvae of the same species, Amblystoma tigrinum, in other parts of North America, and this difference must be attributed to the inherited effect of the conditions.
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