Popular Science Monthly/Volume 77/November 1910/The Continuity of Development
| THE CONTINUITY OF DEVELOPMENT |
By Dr. T. WAYLAND VAUGHAN
U. S. GEOLOGICAL SURVEY
AS nearly every one now admits the validity of the arguments in favor of the derivations of the existing groups of organisms from previous somewhat different organisms through the operation of natural causes, I will not enter upon a discussion of the truth of the theory of organic evolution, nor will I present the results of phylogenetic studies. We will assume evolution to be true, and having made this assumption, the theories of the process and the underlying causes may be discussed.
Only two theories of the process of evolution seem to me possible: (1) Darwin's theory of gradual transformation, or the origin of new species by the gradual augmentation through successive generations of the difference between progeny and ancestors; (2) that brought particularly into prominence by de Vries, the theory of saltation, called by him mutation, according to which the progeny differs definitely, without intergradation, from the parents, and the difference is perpetuated by heredity. There are two theories of the cause of evolution. According to the first, that of Weismann, the cause is within the organisms themselves, new kinds being produced by an inherent tendency to vary, this tendency being due to differences in the germ cells of the two parents; the second theory attributes the cause to the action of the environment on the organisms inhabiting it.
The fundamental problems of evolution can then be resolved into two questions. Is evolution through gradual divergence from the parental type, or by saltation; and is it caused merely by the differences in the parental germ-plasms or is heritable variation produced by the environment acting on the organisms?
As we are all paleontologists, the question may appropriately be put, what light can paleontology throw on these problems? It may perhaps render some assistance in deciding between gradual transformation and saltation, when superimposed conformable beds contain sufficiently abundant faunas, and perhaps the Tertiary marine formations of our southern states will yield important results when studied in proper detail. Dr. Dall has already traced more or less completely the genealogy of some of the species, and I have noticed certain series of species—the group of Corbula fossata, C. oniscus, C. wailesiana, etc., being one of them—deserving thorough study, but the paleontologic work known to me has not as yet been done with the requisite detail to form the basis of an opinion. The principal contribution to our general knowledge of the evolution of organisms that paleontology can make, however, is, I believe, in tracing out phylogenetic lines, and I believe the discovery of the processes and causes of evolution must rest with the experimental biologist. During the past few years very important experimental investigations have been made by several men, and I venture to refer to their results, as I regard paleontology as only an aspect of biology, and think the students in that field should utilize the information gleaned in others.
In the study of variation it has been shown that the selection of fluctuating variations does not carry the species beyond a certain limit, or the extent of the variation is limited, leading to the conclusion that new species can not be produced by this method. I may here refer to ecological surveys and the unreliability of conclusions reached by such researches. Dr. Merriam several years ago presented a paper "Is Mutation a Factor in the Evolution of the Higher Vertebrates?" in which he announced the conclusion that it was not. A critical examination of Dr. Merriam's data showed he had not sufficient information on which to base such a conclusion. His data possess value for the study of evolution in that they indicate material that may be profitably investigated by the experimental method. Attention should also be called to the probable insufficiency of conclusions reached by studying material from successive geologic horizons. For instance, suppose that two usually distinct forms are connected by intermediates. There are no means of ascertaining whether the intermediates represent transition stages between the two forms or are examples of blended hybridism.
That new species may originate through saltation is rather definitely proved; but that it is the only process is not established.
To consider the causes of the origin of new forms: That new forms should originate from the old without the action of some new influence seems to me impossible. The circle of possible combinations of already existent characters could not be transcended, and there would result by crossing only all the combinations possible within definite limits; this would be especially obvious if the de Vries hypothesis of unit-characters be true. Many experiments to determine the influence of various physical factors on individuals showed only somatic changes not of heritable nature and the data accumulated seem definitely to prove that somatic changes, or acquired characters induced through the soma, are not inherited.
Weismann made a great contribution to the progress of biology by focusing attention on the germ cells, and although many of his speculations may be discarded, he was a great stimulator of thought. The work of MacDougal and Tower seems to show how the environment may act on the individual through the germ-cells and induce permanent changes in the progeny.
MacDougal has experimented with species of evening-primroses, by injecting salt solutions into the seed capsules, and summarizes his conclusions in two paragraphs:[1]
The action of reagents having an osmotic and a chemical effect has resulted in the induction of mutants in the progeny of Raimannia odorata and Œnothera biennis. The mutants thus induced have been tested to the second and third generation and found to come true to their newly assumed characters.
The induction of mutants by the action of reagents is a conclusive demonstration of the fact that hereditary characters may be altered by external forces acting directly upon the reproductive mechanism. The action of the reagents used experimentally is simulated by many conditions occurring in nature.
Tower has conducted a series of experiments on species of beetles belonging to the genus Leptinotarsa. He endeavored to influence development by the conditions of moisture and temperature during the germinal stages, and induced changes that were perpetuated in the offspring, the changed offspring at least in some instances mendelizing with the parent species. He presents his conclusions in the following words:[2]
A careful consideration of the various lines of experimentation recorded and of the pedigree cultures and the data from observations in nature irresistibly forces one to the conclusion that in these beetles the only variations of permanence are germinal, and that evolution is through germinal variations. Those germinal variations which arise in nature are permanent and the same variations, of the same degree of permanence, are produced in experiment. The diverse kinds of evidence produced in this and in preceding chapters all go to show that under varying conditions of their surroundings these beetles vary, and that as they become more and more extreme an increasing percentage of striking, permanent variations is found; and as I have just shown, it is possible in experiment to produce in this same way a variety of permanent modifications. From all this evidence, however, there nowhere appears the least trace of a suggestion of any specific action of the conditions of existence, but everywhere there appears only the action of environment as a stimulus, while the response is entirely determined by the organism. All of these variations of purely temporary and of permanent kinds resolve themselves into responses of the organism to the stimuli of its environment, but the nature of the response is entirely determined within the organisms. It is true that different intensities of the same stimuli call forth different responses, but, as is shown in the chapter on 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:[3]
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.
- ↑ "Mutations, Variations and Relationships of the Œnotheras," Carnegie Institution of Washington, No. 81. p. 90, 1907.
- ↑ "An Investigation of Evolution in Chrysomelid Beetles of the Genus Leptinotarsa" Carnegie Institution of Washington, Publication No. 48, p. 295, 1906.
- ↑ "The Dynamics of Living Matter," p. 3, 1906.