Gregor Mendel’s work on peas was unknown to Charles Darwin. To account for variation in individuals and the passing on of traits from one generation to the next, Darwin and many other scientists developed theories of heredity around this time.
In his 1868 book titled Variation of Animals and Plants Under Domestication, Darwin proposed a mechanism of heredity called “pangenesis”. Pangenesis was Darwin’s hypothesis to explain the transmission of characters from one generation to the next, the inheritance of acquired characteristics, limb regeneration, and many other phenomena. While this theory is considered false today (in part, due to Galton’s experiments), it is to Darwin’s credit that he called this a “provisional hypothesis” because he did not have definitive proof.
Simply put, Darwin argued that hereditary particles called “gemmules” were responsible for the characteristics that organisms developed in their life. They were passed on from the parents through the sperm and egg in sexually reproducing organisms. Not all gemmules resulted in feature expression in the organism; some would remain hidden in the cells. Consequently, characteristics of grandparents might be passed on to grandchildren, and the parents may not exhibit those traits. Darwin thought these gemmules were diffused through the body from cell to cell, eventually ending up in the gonads, where sperm and eggs form. This was how Darwin explained heredity and the variation of traits
When Darwin’s half-cousin and friend Francis Galton read about this theory of pangenesis, he thought that if the gemmules were real, then they would be transmitted through the blood. This is a fair assumption since blood circulates through the tissues of the body, eventually passing through the gonads.
Galton’s Experiments on Rabbits
Toward the end of 1869 Galton devised a simple experiment to test Darwin’s theory. He took rabbits of several different breeds and transfused blood between them. Mixed-blood Rabbits were then bread with other mixed-blood rabbits of the same breed to see if the effects of the other breed’s gemmules could be detected in their offspring.
Galton’s first experiment involved three ounces of blood from common lop-eared rabbit, which was transfused it into silver grey rabbits. This amount of rabbit blood would be “proportionally equivalent to six pounds in a man” ( 1871, p 269). The mixed-blood silver grey rabbits were then bred. They produced 36 offspring; 35 were the standard silver grey and one was silver grey with a white foot. Darwin and Galton were hopeful that the white foot was an indication that the theory of gemmules was valid, but they needed more experiments to confirm this.
Galton did a second experiment that required more delicate technique. In this experiment, the carotid arteries of common rabbits and silver grey rabbits were connected. The cross circulation of the blood was then allowed between the rabbits for some time, over 30 minutes in some cases. These rabbits (common and silver grey) were then bred with their kind. The silver grey rabbits produced 50 offspring and the common rabbits produced 39. Of the 89 total offspring, all but one was exactly like its parents. The one anomaly could be expected because of Galton’s particular group of rabbits.
Galton published the results of these experiments in 1871 in a journal called Annals and Magazine of Natural History. He reported that Darwin’s hypothesis of pangenesis was likely false.
Darwin’s Response to Galton
Darwin was not happy about Galton’s negative findings for his hypothesis, and in 1871, he responded in the journal Nature. Darwin thought that Galton was too quick to conclude that the gemmules would circulate in the blood of animals. In his original book, Darwin had not said anything about the circulation of the gemmules in the blood. Rather, he thought the gemmules might pass from cell to cell by other means. By de-emphasizing the role of the blood in gemmule circulation, Darwin effectively neutralized Galton’s experiments from entirely disproving his theory of pangenesis.
Darwin ended his response to Galton with the following words: “I think everyone will admit that his [Galton’s] experiments are extremely curious, and that he deserves the highest credit for his ingenuity and perseverance. But it does not appear to me that pangenesis has, as yet, received its death blow. ”(1871, p. 503)
Sources:
The Complete Works of Charles Darwin
Francis Galton: Pioneer of Heredity and Biometry By M.G. Bulmer
The Academy, Issue 2, J. Murray, 1871, p. 269
Mark Ulett - Mark Ulett is a graduate student at Arizona State University. He studies the history of biology and evolution.
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