Question

The French taxonomist who, in 1809, proposed the first testable evolutionary theory that organisms using body parts repeatedly would increase their abilities was

Answer 1

Jean Lamarck was the first scientist to give this theory of use and misuse of organs. This is popularly know as Lamarckism .
this theory says that if organism utilise one part of their body a lot, there will be an increase in its size which will be a heritable change. One very good example of this theory is

1.long necks if giraffes. Lamarck suggested that on continuous elongation of necks by giraffes to eat the leaves on the top of the trees, their necks will elongated due to heritable changes.

2. eyes of cave animals. Most of the cave dwelling animals are blind and have heightened sense of hearing. This could attribute to their disuse of the organ( the eye).

Appealing in the nature but this theory doesn't answer all the questions. For example, if this theory was correct then the son of a boxer should have naturally built good muscles. It is proven now that the qualities we acquire in life time are not passed on to the generation if there is no direct involvement of genes and their modifications.

change in somatic cells are not heritable. Only germ cells are transferred down the species.

After Lamarckism came natural selection theory by Charles Darwin. This theory believes that there are certain mutations in the population and the most beneficial ones are selected by natural selection over time. So according to Darwinism, the long necks of giraffes are due to natural selection of the genes for long necks over an evolutionary period of time. The long necks of giraffes offered higher survival rates so, it got selected.

lamarckism gave a good idea about the approach to deal with the evolution. Many other theories were build with the development of genetics.