two events separated by one hundred years mark scientific revolutions that symbolise, more than anything else, the web of life. The first event was the publication of Charles Darwin's The Origin of Species in 1859. In this book Darwin pointed out that life on earth was a product of evolution.

The book advances a theory for how evolution takes place. This theory is known as natural selection. According to it, some organisms leave behind more offspring than others because they have different heritable traits. Such traits eventually spread through a population.

Natural selection was immediately hailed as the correct explanation for evolution. But it had gaps. Darwin was unable to explain what causes variation, what makes one individual differ from another. He was unable to explain the mechanism of heredity