Page:Popular Science Monthly Volume 82.djvu/51

Rh by Darwin. In the light of careful experiment, however, it has been largely discredited. The verdict of "not proven" has been pronounced against it, and many biologists would go even further and claim with T. H. Morgan that the theory was "unnecessary." Yet, not content with such a verdict, a small number of workers have persisted in their attempts to establish the theory of acquired characters as one of the essential factors of evolution.

Recently the discussions of evolution have begun to take a new turn. The old attempt to find one single all-important factor is being abandoned for a broader point of view that allows the possibility of many factors, some of them perhaps still unknown. V. L. Kellogg has pointed out the smallness of the number of observed mutations on which to base a comprehensive theory. Castle, a strong believer in mutation and unit characters, has affirmed his belief in the efficacy of selection in the production of new forms. Nowhere in the literature of the last year or two can be found any very dogmatic claim for a single allimportant factor which will serve as the basis for all kinds of evolution.

In this new atmosphere Lamarck's theory again receives serious attention, but not in its old form. To-day no one ventures to cite such examples as Spencer's famous illustration of the puppy that inherited from its mother the trick of begging for food. Such experiments as breeding away the wings of flies in small tubes, or breeding away the eyes of flies in dark chambers, attract but little attention. No great biologist is giving much time to experiments testing the inheritance of mutilations. On the other hand, there are many experiments to test the inherited effect of starvation, to test the effect of the application of chemicals directly to the germ plasm, and to test the effect of the application of extremes of temperature to animals with ripe germ cells. Several investigators have shrewdly seen the value of working with plastic types of animals like the amphibians, which present striking examples of dimorphism such as are found in axolotyl, Diemyctylus and various frog tadpoles. In this field a prominent worker is Kammerer, a representative of a school of experimental evolution in Vienna.

A short summary will be given of his researches on toads, tree frogs and salamanders. A few selected experiments will show very well the nature of the most recent work on the inheritance of acquired characters.

Kammerer in his work on the toad, Alytes, tried to prolong the tadpole stage until sexual maturity. He exposed the young tadpoles to a number of conditions such as darkness, cold, perfectly still water, each of which acting by itself tended to prolong the larval period. By exposing tadpoles to all of these conditions acting at the same time, he succeeded in producing one sexually mature female with the usual form of a tadpole but with mouth, legs and sexual organs of an adult toad. This one example was mated to a normal male. The progeny