coloration, the response is entirely determined within the organism, which is adjusted to different intensities of stimuli and reacts according to its own method and on the basis of its own constitution, there being no specific reaction called forth by a given stimulus.
I conclude in the light of these experiments that the production of heritable variations, slight or extreme, represents in these beetles the response of the germ plasm to stimuli. In my experiments these stimuli were external, but there is no a priori reason why they might not also be internal.
I desire also to call your attention to some remarks by Loeb:[1]
It is obvious that no theory of evolution can be true which disagrees with the fundamental facts of heredity. It is the merit of de Vries to have shown that a mutation of species can be directly observed in certain groups of plants, and he has further shown that the changes occur by jumps, not gradually. This fact harmonizes with the consequence to be drawn from Mendel's experiments that each individual characteristic of a species is represented by an individual determinant in the germ. This determinant may be a definite chemical compound. The transition or mutation from one form into another is therefore only possible through the addition or disappearance of one or more of the characteristics of determinants. If this view can be applied generally, it is just as inconceivable that there should be gradual variation of an individual characteristic and intermediary stages between two elementary mutations, as that there should be gradual transitions between one alcohol and its next neighbor in a chemical series.
To summarize my own opinions on this subject:
1. I think it very doubtful if paleontology can make any especially valuable contribution to our knowledge of the process or causes of the evolution of organisms, and that this field must be surrendered to the experimental biologist.
2. The results of experimental work indicate that the process is not by the gradual transformation of species, but by saltation. However, the former method has not been shown impossible.
3. Experimental investigations also indicate that the cause of evolution is by the environment acting on an organism capable of responding to it.
4. The causes of evolution are chemical in their nature, and the aid of the chemist is necessary for their thorough elucidation.
- ↑ "The Dynamics of Living Matter," p. 3, 1906.
