Page:Popular Science Monthly Volume 76.djvu/330

326

case of my tadpoles, it seemed that the completion of development was due to my imprudently feeding them in spring of 1891 on the very substantial flesh of their congeners; and in the course of some three weeks at most the limbs were evolved, the long tail disappeared gradually, the very color and appearance of the skin underwent considerable change, and my superannuated tadpoles became toads at last.

Modifications of this sort are shown in the experiments of Kammer. Salamandra atra acquires yellowish white spots through higher temperature and moisture. Low temperature and dry conditions make Salamandra maculosa more black in color with a diminution of the yellow spots.

The amphibians show good powers of regeneration, but their faculty in this direction is neither more nor less than is warranted by their position in the phylogenetic scale. This is well indicated by Przibram. In their younger and lower types they belong in the fourth class of animals, or those which can regenerate not only the tail, but also limbs, sense organs and other portions of the body as long as the central nervous system is not removed. More adult and higher types lose some of this power and then may regenerate the tail only or only certain tissues.

The general conclusion from all these experiments on amphibians involving artificially induced changes in the condition of temperature, light, gravity, salt, electricity, oxygen and food is that when we arrive as high in the phylogenetic scale as the amphibians very little can be done to permanently modify the predetermined forces of the germplasm. As among fishes the most striking permanent changes are concerned with the pigmentation.

In powers of regeneration the reptiles show their higher phylogenetic rank. The lizard can regenerate the tail, but not the limbs. It is interesting that the new tail is not composed of bones, but is a cartilaginous tube attached to the half of the broken seventh caudal vertebra. Snakes and turtles will not regenerate their tails.

Modification experiments, other than those concerning regeneration, are not numerous or suggestive. I find only reference to the well-known pigmental response of the chameleon.

As we ascend the scale we not only find that modification experiments are less striking in character, but also find fewer experiments recorded. Noteworthy modifications of birds are almost entirely