Darwin had believed that Lamarckism might have a role in evolution. Weismann, a later follower of Darwin, argued that natural selection alone was sufficient and plausible as a driving force for evolution. For Lamarckism to work, in order for changes to be inherited, every part of the body must be involved in making sperm and eggs. This was widely assumed to be the case. Weismann believed that the body was constructed according to information stored in the ‘germ plasm’, which was then replicated to produce offspring. We now know that this is the case; the ‘germ plasm’ is the DNA within the reproductive organs. When we are conceived, the genetic material from our parents is combined to produce our genome, which then directs the construction of our bodies. This DNA cannot be changed in our lifetime.
Weismann demonstrated the failure of Lamarckism in a famous experiment. He cut off the tails of mice and allowed them to reproduce. He repeated this for many generations. There was no change in the average length of tail. This showed that tailless mice still had the information to produce tails, meaning that the tails were not involved in making new mice. This implied that Weismann’s theory was correct and that Lamarckism could not occur. Unfortunately, this did not convince many neo-Lamarckians. Forced to choose, most instead rejected natural selection. By the start of the 20th century, Darwinists were in a minority.
Another rival theory developed at around this time. This was orthogenesis, which proposed that evolution was directed by internal forces. This predicted that new variations appear non-randomly and are directed towards a particular end. Orthogenesis, however, could not explain how some traits exist that are not fully adaptive, or that in the long run end up driving a species to extinction. One example was the Irish Elk, which is thought to have gone extinct because its antlers grew too large, as they imposed a heavy energetic cost and made the species more vulnerable to predators. The theory doesn’t explain why traits which are costly would exist. It also did not explain how complex structures appeared in the first place, just how they changed. Orthogenesis lingered on for a while, complicating the evolution debate even further.