One of the popular explanations for species diversity around Darwin's time came from Jean-Baptiste Lamarck. Lamark's theory of acquired characteristics was so popular that it is today often known as 'Lamarkism', but scientists can find no evidence to support this thoery any more. In brief, Darwin, during his journey around the world on The Beagle made detailed observations of the animals and to a lesser extent the plants he found. On his return to England, he pondered these observations for many years, trying to make sense of them and explaining the patterns he had observed. It was many years before he published the book which still underpins what scientists now believe about species, their evolution and their relationships.
All species produce far more offspring than are needed to replace the parents.
Therefore most offspring must die without reproducing. Within a population individuals vary . No two are exactly alike. If the survivors are not the 'average' for the population, then over many generations, the population will slowly change. The origins of variation within a population are mutation and random assortment of chromosomes during meiosis .