Evolution
Previous to the publication of Darwin’s “Origin of Species” in 1859, American zoologists were generally inclined toward special creation, in spite of the evidences for evolution which had been presented by Erasmus Darwin, Buffon, Lamarck, and Geoffroy St.-Hilaire. This attitude of mind continued for some years after the publication of the natural selection theory of Darwin and Wallace. This was in part due to the powerful influence of Louis Agassiz and others who bitterly opposed the Darwinian theory. The influence of Asa Gray in gaining a general acceptance for this theory is explained in the following chapter.
A modified Lamarckian doctrine was widely accepted in the last quarter of the century, due largely to the influence of Cope, Hyatt and Packard. The inheritance of “acquired characters” demanded by this theory seems incompatible with the discoveries of recent times, so that “to-day the theory has few followers amongst trained investigators, but it still has a popular vogue that is wide-spread and vociferous.”183
The origin of new varieties and species by accidental and fortuitous modifications (mutations) of the germplasm is now the most widely accepted theory of evolution.
Some of the most important discoveries regarding the origin of new forms have been recently made by Morgan and his pupils. From a stock of the common fruit fly (Drosophila ampelophila) more than 125 new types have arisen within six years. Each of these types breeds true. “Each has arisen independently and suddenly. Every part of the body has been affected by one or another of these mutations.” To arrange these mutations arbitrarily into graded series would give the impression of an evolutionary series, but this is directly contrary to the known facts concerning their origin, for each mutation “originated independently from the wild type.” “Evolution has taken place by the incorporation into the race of those mutations that are beneficial to the life and reproduction of the individual.” This evolutionary process is usually accompanied by the elimination of those forms which have remained stable or which have developed adverse mutations.
A question that is being vigorously debated at this time concerns the possible effects of selection on the hereditary factors. Are the genes fixed both qualitatively and quantitatively or does a given gene vary in potency under different conditions and in different individuals? In the former case selection can only separate the existing genes into separate pure strains. But if the gene be quantitatively variable, then selection will result in the establishment of new types.
Castle has long stoutly maintained the effect of such selection, and his forces have recently been augmented by Jennings. The experimental work now in process will doubtless yield a decisive answer.